diff --git a/lib/doc/workbook.js b/lib/doc/workbook.js
index a6e4de920..9c2f3e4f7 100644
--- a/lib/doc/workbook.js
+++ b/lib/doc/workbook.js
@@ -29,6 +29,8 @@ class Workbook {
     this.media = [];
     this.pivotTables = [];
     this._definedNames = new DefinedNames();
+    // Non-range defined names (LAMBDA, LET, etc.) that cannot be stored in CellMatrix
+    this._formulaDefinedNames = [];
   }
 
   get xlsx() {
@@ -161,7 +163,7 @@ class Workbook {
       properties: this.properties,
       worksheets: this.worksheets.map(worksheet => worksheet.model),
       sheets: this.worksheets.map(ws => ws.model).filter(Boolean),
-      definedNames: this._definedNames.model,
+      definedNames: this._definedNames.model.concat(this._formulaDefinedNames),
       views: this.views,
       company: this.company,
       manager: this.manager,
@@ -221,7 +223,9 @@ class Workbook {
       worksheet.model = worksheetModel;
     });
 
-    this._definedNames.model = value.definedNames;
+    const allDefinedNames = value.definedNames || [];
+    this._formulaDefinedNames = allDefinedNames.filter(dn => dn.formula !== undefined);
+    this._definedNames.model = allDefinedNames.filter(dn => dn.formula === undefined);
     this.views = value.views;
     this._themes = value.themes;
     this.media = value.media || [];
diff --git a/lib/xlsx/xform/book/defined-name-xform.js b/lib/xlsx/xform/book/defined-name-xform.js
index 9ed8fec0d..c0c5fd5f8 100644
--- a/lib/xlsx/xform/book/defined-name-xform.js
+++ b/lib/xlsx/xform/book/defined-name-xform.js
@@ -11,7 +11,8 @@ class DefinedNamesXform extends BaseXform {
       name: model.name,
       localSheetId: model.localSheetId,
     });
-    xmlStream.writeText(model.ranges.join(','));
+    // Non-range defined names (e.g. named LAMBDAs) are preserved verbatim in formula field
+    xmlStream.writeText(model.formula !== undefined ? model.formula : model.ranges.join(','));
     xmlStream.closeNode();
   }
 
@@ -32,10 +33,13 @@ class DefinedNamesXform extends BaseXform {
   }
 
   parseClose() {
-    this.model = {
-      name: this._parsedName,
-      ranges: extractRanges(this._parsedText.join('')),
-    };
+    const text = this._parsedText.join('');
+    const ranges = extractRanges(text);
+    this.model = {name: this._parsedName, ranges};
+    // Preserve non-range content (e.g. LAMBDA, LET, or other formula expressions) verbatim
+    if (ranges.length === 0 && text.trim().length > 0) {
+      this.model.formula = text;
+    }
     if (this._parsedLocalSheetId !== undefined) {
       this.model.localSheetId = parseInt(this._parsedLocalSheetId, 10);
     }
@@ -53,6 +57,21 @@ function isValidRange(range) {
 }
 
 function extractRanges(parsedText) {
+  // A defined-name value is a formula expression (e.g. LAMBDA(x,x*2), LET, OFFSET)
+  // rather than a range list when it contains a '(' that is NOT inside a single-quoted
+  // sheet name. Sheet names with parentheses look like 'Data (2026)'!$A$1 — the '('
+  // appears between an odd number of preceding single quotes (i.e. inside a quotation).
+  // This heuristic avoids splitting LAMBDA/LET bodies whose comma-delimited tokens can
+  // accidentally pass isValidRange.
+  const firstParen = parsedText.indexOf('(');
+  if (firstParen !== -1) {
+    const singleQuotesBefore = (parsedText.slice(0, firstParen).match(/'/g) || []).length;
+    // If the number of single quotes before '(' is even (including zero), the '(' is
+    // outside any quoted sheet name — treat the whole value as a formula expression.
+    if (singleQuotesBefore % 2 === 0) {
+      return [];
+    }
+  }
   const ranges = [];
   let quotesOpened = false;
   let last = '';
diff --git a/spec/integration/pr/lambda-defined-name.spec.js b/spec/integration/pr/lambda-defined-name.spec.js
new file mode 100644
index 000000000..c925334e1
--- /dev/null
+++ b/spec/integration/pr/lambda-defined-name.spec.js
@@ -0,0 +1,111 @@
+// Integration test: named LAMBDA defined names must survive an ExcelJS round-trip.
+// Modern Excel (2024+) supports workbook-level LAMBDA definitions like:
+//   MyDouble = LAMBDA(x, x*2)
+// Prior to this fix, DefinedNamesXform.extractRanges() would silently discard
+// any definedName whose value is not a valid range address, losing the LAMBDA.
+
+const JSZip = require('jszip');
+
+const ExcelJS = verquire('exceljs');
+
+const WORKBOOK_XML_WITH_LAMBDA = `<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<workbook xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"
+          xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships">
+  <fileVersion appName="xl" lastEdited="7" lowestEdited="7" rupBuild="22228"/>
+  <workbookPr defaultThemeVersion="166925"/>
+  <bookViews>
+    <workbookView xWindow="0" yWindow="0" windowWidth="28800" windowHeight="12420"/>
+  </bookViews>
+  <sheets>
+    <sheet name="Sheet1" sheetId="1" r:id="rId1"/>
+  </sheets>
+  <definedNames>
+    <definedName name="MyDouble">LAMBDA(x,x*2)</definedName>
+    <definedName name="MyAdd">LAMBDA(x,y,x+y)</definedName>
+    <definedName name="NormalRange">Sheet1!$A$1:$B$2</definedName>
+  </definedNames>
+  <calcPr calcId="191028"/>
+</workbook>`;
+
+const SHEET_XML = `<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"
+           xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships">
+  <sheetData>
+    <row r="1"><c r="A1" t="n"><v>1</v></c></row>
+  </sheetData>
+</worksheet>`;
+
+const RELS_XML = `<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">
+  <Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet" Target="worksheets/sheet1.xml"/>
+</Relationships>`;
+
+const ROOT_RELS_XML = `<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">
+  <Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument" Target="xl/workbook.xml"/>
+</Relationships>`;
+
+const CONTENT_TYPES_XML = `<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<Types xmlns="http://schemas.openxmlformats.org/package/2006/content-types">
+  <Default Extension="rels" ContentType="application/vnd.openxmlformats-package.relationships+xml"/>
+  <Default Extension="xml" ContentType="application/xml"/>
+  <Override PartName="/xl/workbook.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml"/>
+  <Override PartName="/xl/worksheets/sheet1.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml"/>
+</Types>`;
+
+async function buildXlsxWithLambda() {
+  const zip = new JSZip();
+  zip.file('[Content_Types].xml', CONTENT_TYPES_XML);
+  zip.file('_rels/.rels', ROOT_RELS_XML);
+  zip.file('xl/workbook.xml', WORKBOOK_XML_WITH_LAMBDA);
+  zip.file('xl/_rels/workbook.xml.rels', RELS_XML);
+  zip.file('xl/worksheets/sheet1.xml', SHEET_XML);
+  return zip.generateAsync({type: 'nodebuffer'});
+}
+
+async function roundTrip(inputBuffer) {
+  const wb1 = new ExcelJS.Workbook();
+  await wb1.xlsx.load(inputBuffer);
+  const outBuffer = await wb1.xlsx.writeBuffer();
+  const wb2 = new ExcelJS.Workbook();
+  await wb2.xlsx.load(outBuffer);
+  return {wb2, outBuffer};
+}
+
+describe('Named LAMBDA defined names', () => {
+  let inputBuffer;
+
+  before(async () => {
+    inputBuffer = await buildXlsxWithLambda();
+  });
+
+  it('round-trips LAMBDA defined names verbatim without dropping them', async () => {
+    const {outBuffer} = await roundTrip(inputBuffer);
+    const zip = await JSZip.loadAsync(outBuffer);
+    const workbookXml = await zip.file('xl/workbook.xml').async('string');
+
+    expect(workbookXml).to.include('MyDouble');
+    expect(workbookXml).to.include('LAMBDA(x,x*2)');
+    expect(workbookXml).to.include('MyAdd');
+    expect(workbookXml).to.include('LAMBDA(x,y,x+y)');
+  });
+
+  it('preserves normal range defined names alongside LAMBDA definitions', async () => {
+    const {outBuffer} = await roundTrip(inputBuffer);
+    const zip = await JSZip.loadAsync(outBuffer);
+    const workbookXml = await zip.file('xl/workbook.xml').async('string');
+
+    expect(workbookXml).to.include('NormalRange');
+    expect(workbookXml).to.include('Sheet1!$A$1:$B$2');
+  });
+
+  it('preserves LAMBDA formula text exactly as stored', async () => {
+    const {outBuffer} = await roundTrip(inputBuffer);
+    const zip = await JSZip.loadAsync(outBuffer);
+    const workbookXml = await zip.file('xl/workbook.xml').async('string');
+
+    const lambdaMatch = workbookXml.match(/name="MyDouble"[^>]*>([^<]*)<\/definedName>/);
+    expect(lambdaMatch).to.not.be.null();
+    expect(lambdaMatch[1]).to.equal('LAMBDA(x,x*2)');
+  });
+});
diff --git a/spec/unit/xlsx/xform/book/defined-name-xform.spec.js b/spec/unit/xlsx/xform/book/defined-name-xform.spec.js
index 27a6b10ca..36580064d 100644
--- a/spec/unit/xlsx/xform/book/defined-name-xform.spec.js
+++ b/spec/unit/xlsx/xform/book/defined-name-xform.spec.js
@@ -39,9 +39,40 @@ const expectations = [
     },
     preparedModel: {name: 'foo', ranges: []},
     xml: '<definedName name="foo">"OFFSET($A$10;0;0;0;1)"</definedName>',
-    parsedModel: {name: 'foo', ranges: []},
+    parsedModel: {name: 'foo', ranges: [], formula: '"OFFSET($A$10;0;0;0;1)"'},
     tests: ['parse'],
   },
+  {
+    title: 'Range on sheet name containing parentheses',
+    create() {
+      return new DefinedNameXform();
+    },
+    // Sheet names with '(' must NOT be misclassified as formula expressions.
+    preparedModel: {name: 'Foo', ranges: ["'Data (2026)'!$A$1"]},
+    xml: "<definedName name=\"Foo\">'Data (2026)'!$A$1</definedName>",
+    parsedModel: {name: 'Foo', ranges: ["'Data (2026)'!$A$1"]},
+    tests: ['render', 'renderIn', 'parse'],
+  },
+  {
+    title: 'Named LAMBDA expression',
+    create() {
+      return new DefinedNameXform();
+    },
+    preparedModel: {name: 'MyDouble', ranges: [], formula: 'LAMBDA(x,x*2)'},
+    xml: '<definedName name="MyDouble">LAMBDA(x,x*2)</definedName>',
+    parsedModel: {name: 'MyDouble', ranges: [], formula: 'LAMBDA(x,x*2)'},
+    tests: ['render', 'renderIn', 'parse'],
+  },
+  {
+    title: 'Named LAMBDA with multiple parameters',
+    create() {
+      return new DefinedNameXform();
+    },
+    preparedModel: {name: 'MySum', ranges: [], formula: 'LAMBDA(x,y,x+y)'},
+    xml: '<definedName name="MySum">LAMBDA(x,y,x+y)</definedName>',
+    parsedModel: {name: 'MySum', ranges: [], formula: 'LAMBDA(x,y,x+y)'},
+    tests: ['render', 'renderIn', 'parse'],
+  },
 ];
 
 describe('DefinedNameXform', () => {