diff --git a/src/test/java/com/ironcorelabs/tenantsecurity/kms/v1/LegacyFormatDecryptTest.java b/src/test/java/com/ironcorelabs/tenantsecurity/kms/v1/LegacyFormatDecryptTest.java
new file mode 100644
index 0000000..6505e34
--- /dev/null
+++ b/src/test/java/com/ironcorelabs/tenantsecurity/kms/v1/LegacyFormatDecryptTest.java
@@ -0,0 +1,59 @@
+package com.ironcorelabs.tenantsecurity.kms.v1;
+
+import static org.testng.Assert.assertEquals;
+import java.util.Base64;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.CompletableFuture;
+import com.ironcorelabs.tenantsecurity.TestUtils;
+import org.testng.annotations.Test;
+
+/**
+ * Integration test that decrypts V3-format documents produced by ironcore-alloy in legacy mode.
+ * This confirms that the TSC Java library can decrypt documents written by ironcore-alloy's
+ * legacy_tsc_write_format option.
+ *
+ * The hardcoded EDEK and encrypted field fixtures come from
+ * ironcore-alloy/tests/legacy_format.rs and were encrypted with tenant-gcp-l.
+ */
+@Test(groups = {"local-integration"})
+public class LegacyFormatDecryptTest {
+  private static final String TENANT_ID = "tenant-gcp-l";
+  private static final String API_KEY = "0WUaXesNgbTAuLwn";
+
+  // V3 EDEK from ironcore-alloy legacy format fixtures
+  private static final String V3_EDEK =
+      "CsABCjBouxrw3TFZtSO6cgPOM15ewNFH8uqIU+ordPNLK/M7vS7qihZlJVJNMnEQevFeQ18Q/wMYqg0iDKj2tv0ToheETqmBeyp4CnYKcQokAKUEZIf9Qdt+hRqFMjVQKP0EHlmWMGeU6tQs0bzmrl69vWE4EkkA3PhOjPCtLSjyH9Ds02CuqKTAl6tgBxadfFeWp9JMY059IZN6Gj+qfjT2vPdtWQR0NAhFPN3Ex1FXpqX+NNTcz59jll+2c0eLEP8D";
+
+  // V3 encrypted field bytes from ironcore-alloy legacy format fixtures
+  private static final String V3_ENCRYPTED_FIELD =
+      "A0lST04ALgoc0x0jEo+VjaJpWEgrC2u//30unDURXl37Y2UbYBoOCgx0ZW5hbnQtZ2NwLWyzvlmZt+wuFmRVkkCBONrqkr9kAC/iRF+Mp8i5uRyj";
+
+  private static final byte[] EXPECTED_PLAINTEXT = new byte[] {1, 2, 3};
+
+  private CompletableFuture<TenantSecurityClient> getClient() {
+    Map<String, String> envVars = System.getenv();
+    String tspAddress = envVars.getOrDefault("TSP_ADDRESS", TestSettings.TSP_ADDRESS);
+    String tspPort =
+        TestUtils.ensureLeadingColon(envVars.getOrDefault("TSP_PORT", TestSettings.TSP_PORT));
+    String apiKey = envVars.getOrDefault("API_KEY", API_KEY);
+    return TestUtils.createTscWithAllowInsecure(tspAddress + tspPort, apiKey);
+  }
+
+  public void decryptV3LegacyFormatFromAlloy() throws Exception {
+    Map<String, String> envVars = System.getenv();
+    String tenantId = envVars.getOrDefault("TENANT_ID", TENANT_ID);
+    DocumentMetadata metadata = new DocumentMetadata(tenantId, "legacyFormatTest", "integration");
+
+    byte[] encryptedFieldBytes = Base64.getDecoder().decode(V3_ENCRYPTED_FIELD);
+    Map<String, byte[]> encryptedFields = new HashMap<>();
+    encryptedFields.put("field", encryptedFieldBytes);
+
+    EncryptedDocument encryptedDoc = new EncryptedDocument(encryptedFields, V3_EDEK);
+
+    PlaintextDocument result = getClient()
+        .thenCompose(client -> client.decrypt(encryptedDoc, metadata)).get();
+
+    assertEquals(result.getDecryptedFields().get("field"), EXPECTED_PLAINTEXT);
+  }
+}