diff --git a/modules/nf-core/gcta/reml/environment.yml b/modules/nf-core/gcta/reml/environment.yml
new file mode 100644
index 00000000000..3e22ea7b9f2
--- /dev/null
+++ b/modules/nf-core/gcta/reml/environment.yml
@@ -0,0 +1,7 @@
+---
+# yaml-language-server: $schema=https://raw.githubusercontent.com/nf-core/modules/master/modules/environment-schema.json
+channels:
+  - conda-forge
+  - bioconda
+dependencies:
+  - bioconda::gcta=1.94.1
diff --git a/modules/nf-core/gcta/reml/main.nf b/modules/nf-core/gcta/reml/main.nf
new file mode 100644
index 00000000000..b75bc23c301
--- /dev/null
+++ b/modules/nf-core/gcta/reml/main.nf
@@ -0,0 +1,45 @@
+process GCTA_REML {
+    tag "${meta.id}_${meta2.id}"
+    label 'process_medium'
+
+    conda "${moduleDir}/environment.yml"
+    container "${workflow.containerEngine in ['singularity', 'apptainer'] && !task.ext.singularity_pull_docker_container
+        ? 'https://community-cr-prod.seqera.io/docker/registry/v2/blobs/sha256/46/46b0d05f0daa47561d87d2a9cac5e51edc2c78e26f1bbab439c688386241a274/data'
+        : 'community.wave.seqera.io/library/gcta:1.94.1--9bc35dc424fcf6e9'}"
+
+    input:
+    tuple val(meta), path(grm_files)
+    tuple val(meta2), path(phenotypes_file)
+    tuple val(meta3), path(quant_covariates_file)
+    tuple val(meta4), path(cat_covariates_file)
+
+    output:
+    tuple val(meta), path("*.hsq"), emit: reml_results
+    tuple val("${task.process}"), val("gcta"), eval("gcta --version | sed -En 's/^[*] version v([0-9.]*).*/\\1/p'"), emit: versions_gcta, topic: versions
+
+    when:
+    task.ext.when == null || task.ext.when
+
+    script:
+    def args = task.ext.args ?: ''
+    def prefix = task.ext.prefix ?: "${meta.id}"
+    def qcovar_param = quant_covariates_file ? "--qcovar ${quant_covariates_file}" : ''
+    def covar_param = cat_covariates_file ? "--covar ${cat_covariates_file}" : ''
+    """
+    gcta \\
+        --reml \\
+        --grm ${meta.id} \\
+        --pheno ${phenotypes_file} \\
+        ${qcovar_param} \\
+        ${covar_param} \\
+        --out "${prefix}" \\
+        --thread-num ${task.cpus} \\
+        ${args}
+    """
+
+    stub:
+    def prefix = task.ext.prefix ?: "${meta.id}"
+    """
+    touch "${prefix}.hsq"
+    """
+}
diff --git a/modules/nf-core/gcta/reml/meta.yml b/modules/nf-core/gcta/reml/meta.yml
new file mode 100644
index 00000000000..9b35bdda10d
--- /dev/null
+++ b/modules/nf-core/gcta/reml/meta.yml
@@ -0,0 +1,104 @@
+# yaml-language-server: $schema=https://raw.githubusercontent.com/nf-core/modules/master/modules/meta-schema.json
+name: "gcta_reml"
+description: Run univariate REML heritability estimation with a dense GRM
+keywords:
+  - gcta
+  - genome-wide complex trait analysis
+  - reml
+  - restricted maximum likelihood
+  - grm
+  - genetic relationship matrix
+  - genetics
+tools:
+  - "gcta":
+      description: "Genome-wide Complex Trait Analysis (GCTA) estimates genetic relationships, variance components, and association statistics from genome-wide data."
+      homepage: "https://yanglab.westlake.edu.cn/software/gcta/"
+      documentation: "https://yanglab.westlake.edu.cn/software/gcta/static/gcta_doc_latest.pdf"
+      tool_dev_url: "https://yanglab.westlake.edu.cn/software/gcta/"
+      licence: ["GPL-3.0-only"]
+      identifier: "biotools:gcta"
+input:
+  - - meta:
+        type: map
+        description: |
+          Groovy map containing dense GRM metadata
+          `meta.id` must be the GRM basename passed to `--grm`
+          (for example `plink_simulated_dense` for `plink_simulated_dense.grm.{id,bin,N.bin}`)
+    - grm_files:
+        type: file
+        description: Dense GRM bundle containing sample identifier, binary matrix,
+          and sample-count matrix files
+        pattern: "*.grm.*"
+        ontologies: []
+  - - meta2:
+        type: map
+        description: |
+          Groovy map containing phenotype metadata; should carry the same sample
+          identity as `meta`, for example `[ id:'plink_simulated_dense' ]`
+    - phenotypes_file:
+        type: file
+        description: Phenotype file passed to `--pheno`
+        pattern: "*.{phe,pheno,txt,tsv}"
+        ontologies:
+          - edam: "http://edamontology.org/format_3475"
+  - - meta3:
+        type: map
+        description: |
+          Groovy map containing quantitative covariate metadata; should carry the
+          same sample identity as `meta`, for example `[ id:'plink_simulated_dense' ]`
+    - quant_covariates_file:
+        type: file
+        description: Quantitative covariates file, pass `[]` when absent
+        pattern: "*.{covar,cov,txt,tsv}"
+        ontologies:
+          - edam: "http://edamontology.org/format_3475"
+  - - meta4:
+        type: map
+        description: |
+          Groovy map containing categorical covariate metadata; should carry the
+          same sample identity as `meta`, for example `[ id:'plink_simulated_dense' ]`
+    - cat_covariates_file:
+        type: file
+        description: Categorical covariates file, pass `[]` when absent
+        pattern: "*.{covar,cov,txt,tsv}"
+        ontologies:
+          - edam: "http://edamontology.org/format_3475"
+output:
+  reml_results:
+    - - meta:
+          type: map
+          description: |
+            Groovy map containing dense GRM metadata
+            `meta.id` must be the GRM basename passed to `--grm`
+            (for example `plink_simulated_dense` for `plink_simulated_dense.grm.{id,bin,N.bin}`)
+      - "*.hsq":
+          type: file
+          description: REML result file
+          pattern: "*.{hsq}"
+          ontologies:
+            - edam: "http://edamontology.org/format_2330"
+  versions_gcta:
+    - - "${task.process}":
+          type: string
+          description: The process the version was collected from
+      - "gcta":
+          type: string
+          description: The tool name
+      - "gcta --version | sed -En 's/^[*] version v([0-9.]*).*/\\1/p'":
+          type: eval
+          description: The command used to retrieve the GCTA version
+topics:
+  versions:
+    - - ${task.process}:
+          type: string
+          description: The process the version was collected from
+      - gcta:
+          type: string
+          description: The tool name
+      - "gcta --version | sed -En 's/^[*] version v([0-9.]*).*/\\1/p'":
+          type: eval
+          description: The command used to retrieve the GCTA version
+authors:
+  - "@lyh970817"
+maintainers:
+  - "@lyh970817"
diff --git a/modules/nf-core/gcta/reml/tests/main.nf.test b/modules/nf-core/gcta/reml/tests/main.nf.test
new file mode 100644
index 00000000000..ccb41167d80
--- /dev/null
+++ b/modules/nf-core/gcta/reml/tests/main.nf.test
@@ -0,0 +1,272 @@
+nextflow_process {
+
+    name "Test Process GCTA_REML"
+    script "../main.nf"
+    process "GCTA_REML"
+
+    tag "modules"
+    tag "modules_nfcore"
+    tag "gcta"
+    tag "gcta/reml"
+    tag "gcta/makegrm"
+    tag "gawk"
+    config "./nextflow.config"
+
+    setup {
+        run("GAWK", alias: "GAWK_QUANTITATIVE_PHENOTYPE") {
+            script "../../../gawk/main.nf"
+            process {
+                """
+                input[0] = [
+                    [ id:'QuantitativeTrait' ],
+                    file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated_quantitative_phenoname.phe', checkIfExists: true)
+                ]
+                input[1] = Channel.of('{ print \$1, \$2, \$3 }').collectFile(name:'quantitative_phenotypes.awk')
+                input[2] = false
+                """
+            }
+        }
+
+        run("GAWK", alias: "GAWK_MULTI_PHENOTYPES") {
+            script "../../../gawk/main.nf"
+            process {
+                """
+                input[0] = [
+                    [ id:'QuantitativeTraitMpheno' ],
+                    file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated_quantitative_phenoname.phe', checkIfExists: true)
+                ]
+                input[1] = Channel.of('{ print \$1, \$2, \$3, (\$3 * 0.5) + ((NR % 7) / 10.0) }').collectFile(name:'multi_phenotypes.awk')
+                input[2] = false
+                """
+            }
+        }
+
+        run("GAWK", alias: "GAWK_BINARY_PHENOTYPE") {
+            script "../../../gawk/main.nf"
+            process {
+                """
+                input[0] = [
+                    [ id:'BinaryTrait' ],
+                    file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated_binary_phenoname.phe', checkIfExists: true)
+                ]
+                input[1] = Channel.of('FNR == 1 { next } { print \$1, \$2, \$3 + 1 }').collectFile(name:'binary_phenotypes.awk')
+                input[2] = false
+                """
+            }
+        }
+
+        run("GAWK", alias: "GAWK_QUANTITATIVE_COVARIATES") {
+            script "../../../gawk/main.nf"
+            process {
+                """
+                input[0] = [
+                    [ id:'covariates_quant' ],
+                    file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated_covariates.txt', checkIfExists: true)
+                ]
+                input[1] = Channel.of('FNR == 1 { next } { print \$1, \$2, \$4, \$5 }').collectFile(name:'quantitative_covariates.awk')
+                input[2] = false
+                """
+            }
+        }
+
+        run("GAWK", alias: "GAWK_CATEGORICAL_COVARIATES") {
+            script "../../../gawk/main.nf"
+            process {
+                """
+                input[0] = [
+                    [ id:'covariates_cat' ],
+                    file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated_covariates.txt', checkIfExists: true)
+                ]
+                input[1] = Channel.of('FNR == 1 { next } { print \$1, \$2, \$3 }').collectFile(name:'categorical_covariates.awk')
+                input[2] = false
+                """
+            }
+        }
+
+        run("GCTA_MAKEGRM", alias: "GCTA_MAKEGRM_DENSE") {
+            script "../../makegrm/main.nf"
+            process {
+                """
+                file('plink_simulated.mbfile').text = 'plink_simulated\\n'
+
+                input[0] = [
+                    [ id:'plink_simulated_dense' ],
+                    file('plink_simulated.mbfile'),
+                    [
+                        file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated.bed', checkIfExists: true)
+                    ],
+                    [
+                        file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated.bim', checkIfExists: true)
+                    ],
+                    [
+                        file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated.fam', checkIfExists: true)
+                    ]
+                ]
+                """
+            }
+        }
+    }
+
+    test("homo_sapiens popgen - dense GRM with mpheno selection") {
+        when {
+            params {
+                module_args = '--mpheno 2'
+            }
+            process {
+                """
+                input[0] = GCTA_MAKEGRM_DENSE.out.grm_files
+                input[1] = GAWK_MULTI_PHENOTYPES.out.output.map { meta, pheno -> [[ id:'plink_simulated_dense' ], pheno] }
+                input[2] = GAWK_QUANTITATIVE_COVARIATES.out.output.map { meta, covar -> [[ id:'plink_simulated_dense' ], covar] }
+                input[3] = GAWK_CATEGORICAL_COVARIATES.out.output.map { meta, covar -> [[ id:'plink_simulated_dense' ], covar] }
+                """
+            }
+        }
+
+        then {
+            assertAll(
+                { assert process.success },
+                { assert process.out.reml_results.size() == 1 },
+                { assert process.out.reml_results.get(0).get(0).id == "plink_simulated_dense" },
+                { assert snapshot(sanitizeOutput(process.out)).match() }
+            )
+        }
+    }
+
+    test("homo_sapiens popgen - quantitative phenotype with dense GRM and covariates") {
+        when {
+            process {
+                """
+                input[0] = GCTA_MAKEGRM_DENSE.out.grm_files
+                input[1] = GAWK_QUANTITATIVE_PHENOTYPE.out.output.map { meta, pheno -> [[ id:'plink_simulated_dense' ], pheno] }
+                input[2] = GAWK_QUANTITATIVE_COVARIATES.out.output.map { meta, covar -> [[ id:'plink_simulated_dense' ], covar] }
+                input[3] = GAWK_CATEGORICAL_COVARIATES.out.output.map { meta, covar -> [[ id:'plink_simulated_dense' ], covar] }
+                """
+            }
+        }
+
+        then {
+            assertAll(
+                { assert process.success },
+                { assert process.out.reml_results.size() == 1 },
+                { assert process.out.reml_results.get(0).get(0).id == "plink_simulated_dense" },
+                { assert snapshot(sanitizeOutput(process.out)).match() }
+            )
+        }
+    }
+
+    test("homo_sapiens popgen - quantitative phenotype with dense GRM and no covariates") {
+        when {
+            process {
+                """
+                input[0] = GCTA_MAKEGRM_DENSE.out.grm_files
+                input[1] = GAWK_QUANTITATIVE_PHENOTYPE.out.output.map { meta, pheno -> [[ id:'plink_simulated_dense' ], pheno] }
+                input[2] = [[ id:'plink_simulated_dense' ], []]
+                input[3] = [[ id:'plink_simulated_dense' ], []]
+                """
+            }
+        }
+
+        then {
+            assertAll(
+                { assert process.success },
+                { assert process.out.reml_results.size() == 1 },
+                { assert process.out.reml_results.get(0).get(0).id == "plink_simulated_dense" },
+                { assert snapshot(sanitizeOutput(process.out)).match() }
+            )
+        }
+    }
+
+    test("homo_sapiens popgen - binary phenotype with dense GRM and prevalence") {
+        when {
+            params {
+                module_args = '--prevalence 0.1'
+            }
+            process {
+                """
+                input[0] = GCTA_MAKEGRM_DENSE.out.grm_files
+                input[1] = GAWK_BINARY_PHENOTYPE.out.output.map { meta, pheno -> [[ id:'plink_simulated_dense' ], pheno] }
+                input[2] = [[ id:'plink_simulated_dense' ], []]
+                input[3] = [[ id:'plink_simulated_dense' ], []]
+                """
+            }
+        }
+
+        then {
+            assertAll(
+                { assert process.success },
+                { assert process.out.reml_results.size() == 1 },
+                { assert process.out.reml_results.get(0).get(0).id == "plink_simulated_dense" },
+                { assert snapshot(sanitizeOutput(process.out)).match() }
+            )
+        }
+    }
+
+    test("homo_sapiens popgen - dense GRM defaults to first phenotype") {
+        when {
+            process {
+                """
+                input[0] = GCTA_MAKEGRM_DENSE.out.grm_files
+                input[1] = GAWK_MULTI_PHENOTYPES.out.output.map { meta, pheno -> [[ id:'plink_simulated_dense' ], pheno] }
+                input[2] = GAWK_QUANTITATIVE_COVARIATES.out.output.map { meta, covar -> [[ id:'plink_simulated_dense' ], covar] }
+                input[3] = GAWK_CATEGORICAL_COVARIATES.out.output.map { meta, covar -> [[ id:'plink_simulated_dense' ], covar] }
+                """
+            }
+        }
+
+        then {
+            assertAll(
+                { assert process.success },
+                { assert process.out.reml_results.size() == 1 },
+                { assert process.out.reml_results.get(0).get(0).id == "plink_simulated_dense" },
+                { assert snapshot(sanitizeOutput(process.out)).match() }
+            )
+        }
+    }
+
+    test("homo_sapiens popgen - fails when mpheno index is out of range") {
+        when {
+            params {
+                module_args = '--mpheno 3'
+            }
+            process {
+                """
+                input[0] = GCTA_MAKEGRM_DENSE.out.grm_files
+                input[1] = GAWK_MULTI_PHENOTYPES.out.output.map { meta, pheno -> [[ id:'plink_simulated_dense' ], pheno] }
+                input[2] = GAWK_QUANTITATIVE_COVARIATES.out.output.map { meta, covar -> [[ id:'plink_simulated_dense' ], covar] }
+                input[3] = GAWK_CATEGORICAL_COVARIATES.out.output.map { meta, covar -> [[ id:'plink_simulated_dense' ], covar] }
+                """
+            }
+        }
+
+        then {
+            assertAll(
+                { assert !process.success },
+                { assert process.exitStatus != 0 }
+            )
+        }
+    }
+
+    test("homo_sapiens popgen - quantitative phenotype with dense GRM - stub") {
+        options "-stub"
+
+        when {
+            process {
+                """
+                input[0] = GCTA_MAKEGRM_DENSE.out.grm_files
+                input[1] = GAWK_QUANTITATIVE_PHENOTYPE.out.output.map { meta, pheno -> [[ id:'plink_simulated_dense' ], pheno] }
+                input[2] = [[ id:'plink_simulated_dense' ], []]
+                input[3] = [[ id:'plink_simulated_dense' ], []]
+                """
+            }
+        }
+
+        then {
+            assertAll(
+                { assert process.success },
+                { assert process.out.reml_results.size() == 1 },
+                { assert process.out.reml_results.get(0).get(0).id == "plink_simulated_dense" },
+                { assert snapshot(sanitizeOutput(process.out)).match() }
+            )
+        }
+    }
+}
diff --git a/modules/nf-core/gcta/reml/tests/main.nf.test.snap b/modules/nf-core/gcta/reml/tests/main.nf.test.snap
new file mode 100644
index 00000000000..2f546b1a224
--- /dev/null
+++ b/modules/nf-core/gcta/reml/tests/main.nf.test.snap
@@ -0,0 +1,158 @@
+{
+    "homo_sapiens popgen - quantitative phenotype with dense GRM - stub": {
+        "content": [
+            {
+                "reml_results": [
+                    [
+                        {
+                            "id": "plink_simulated_dense"
+                        },
+                        "plink_simulated_dense.hsq:md5,d41d8cd98f00b204e9800998ecf8427e"
+                    ]
+                ],
+                "versions_gcta": [
+                    [
+                        "GCTA_REML",
+                        "gcta",
+                        "1.94.1"
+                    ]
+                ]
+            }
+        ],
+        "meta": {
+            "nf-test": "0.9.3",
+            "nextflow": "25.10.4"
+        },
+        "timestamp": "2026-05-26T21:47:06.863951978"
+    },
+    "homo_sapiens popgen - dense GRM defaults to first phenotype": {
+        "content": [
+            {
+                "reml_results": [
+                    [
+                        {
+                            "id": "plink_simulated_dense"
+                        },
+                        "plink_simulated_dense.hsq:md5,17d5e79e461b582b0aaba2a40666c8f7"
+                    ]
+                ],
+                "versions_gcta": [
+                    [
+                        "GCTA_REML",
+                        "gcta",
+                        "1.94.1"
+                    ]
+                ]
+            }
+        ],
+        "meta": {
+            "nf-test": "0.9.3",
+            "nextflow": "25.10.4"
+        },
+        "timestamp": "2026-05-26T21:46:19.771868442"
+    },
+    "homo_sapiens popgen - quantitative phenotype with dense GRM and no covariates": {
+        "content": [
+            {
+                "reml_results": [
+                    [
+                        {
+                            "id": "plink_simulated_dense"
+                        },
+                        "plink_simulated_dense.hsq:md5,51c1328c8feb6d53f3984cd58324fed7"
+                    ]
+                ],
+                "versions_gcta": [
+                    [
+                        "GCTA_REML",
+                        "gcta",
+                        "1.94.1"
+                    ]
+                ]
+            }
+        ],
+        "meta": {
+            "nf-test": "0.9.3",
+            "nextflow": "25.10.4"
+        },
+        "timestamp": "2026-05-26T21:45:27.688042039"
+    },
+    "homo_sapiens popgen - quantitative phenotype with dense GRM and covariates": {
+        "content": [
+            {
+                "reml_results": [
+                    [
+                        {
+                            "id": "plink_simulated_dense"
+                        },
+                        "plink_simulated_dense.hsq:md5,17d5e79e461b582b0aaba2a40666c8f7"
+                    ]
+                ],
+                "versions_gcta": [
+                    [
+                        "GCTA_REML",
+                        "gcta",
+                        "1.94.1"
+                    ]
+                ]
+            }
+        ],
+        "meta": {
+            "nf-test": "0.9.3",
+            "nextflow": "25.10.4"
+        },
+        "timestamp": "2026-05-26T21:45:01.211801644"
+    },
+    "homo_sapiens popgen - dense GRM with mpheno selection": {
+        "content": [
+            {
+                "reml_results": [
+                    [
+                        {
+                            "id": "plink_simulated_dense"
+                        },
+                        "plink_simulated_dense.hsq:md5,0a29048e72305f462889481b2dfb94db"
+                    ]
+                ],
+                "versions_gcta": [
+                    [
+                        "GCTA_REML",
+                        "gcta",
+                        "1.94.1"
+                    ]
+                ]
+            }
+        ],
+        "meta": {
+            "nf-test": "0.9.3",
+            "nextflow": "25.10.4"
+        },
+        "timestamp": "2026-05-26T21:44:38.283352845"
+    },
+    "homo_sapiens popgen - binary phenotype with dense GRM and prevalence": {
+        "content": [
+            {
+                "reml_results": [
+                    [
+                        {
+                            "id": "plink_simulated_dense"
+                        },
+                        "plink_simulated_dense.hsq:md5,86672f6a0b3c49b2347d402cb03b8606"
+                    ]
+                ],
+                "versions_gcta": [
+                    [
+                        "GCTA_REML",
+                        "gcta",
+                        "1.94.1"
+                    ]
+                ]
+            }
+        ],
+        "meta": {
+            "nf-test": "0.9.3",
+            "nextflow": "25.10.4"
+        },
+        "timestamp": "2026-05-26T21:45:51.759861217"
+    }
+}
\ No newline at end of file
diff --git a/modules/nf-core/gcta/reml/tests/nextflow.config b/modules/nf-core/gcta/reml/tests/nextflow.config
new file mode 100644
index 00000000000..b77f0e7d528
--- /dev/null
+++ b/modules/nf-core/gcta/reml/tests/nextflow.config
@@ -0,0 +1,9 @@
+params {
+    module_args = ""
+}
+
+process {
+    withName: "GCTA_REML" {
+        ext.args = { params.module_args ?: "" }
+    }
+}