diff --git a/.chronus/changes/fix-apiversion-clientdefaultvalue-multi-service-2026-2-4-8-54-0.md b/.chronus/changes/fix-apiversion-clientdefaultvalue-multi-service-2026-2-4-8-54-0.md
new file mode 100644
index 0000000000..c2b9c25fb2
--- /dev/null
+++ b/.chronus/changes/fix-apiversion-clientdefaultvalue-multi-service-2026-2-4-8-54-0.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+Fixed apiVersion parameter clientDefaultValue missing when an operation group contains operations from different services.
diff --git a/packages/typespec-client-generator-core/src/internal-utils.ts b/packages/typespec-client-generator-core/src/internal-utils.ts
index 33e249ba0e..0323d03bf9 100644
--- a/packages/typespec-client-generator-core/src/internal-utils.ts
+++ b/packages/typespec-client-generator-core/src/internal-utils.ts
@@ -289,28 +289,75 @@ export function parseEmitterName(
   return diagnostics.wrap(language);
 }
 
+/**
+ * Find the service namespace that contains the given operation.
+ * @param services Array of service namespaces
+ * @param operation The operation to find the service for
+ * @returns The service namespace that contains the operation
+ */
+function findServiceForOperation(services: Namespace[], operation: Operation): Namespace {
+  let namespace = operation.namespace;
+  while (namespace) {
+    if (services.includes(namespace)) {
+      return namespace;
+    }
+    namespace = namespace.namespace;
+  }
+  // Fallback to the first service. This can happen when an operation is defined outside
+  // of any service namespace (e.g., in Azure.ResourceManager or other shared namespaces)
+  // and is imported into a client that combines multiple services. In such cases,
+  // we use the first service's api version as the default.
+  return services[0];
+}
+
 /**
  *
  * @param context
  * @param type The type that we are adding api version information onto
+ * @param client The client or operation group that contains the operation
+ * @param operation The operation that contains the api version parameter (needed for multi-service operation groups)
  * @returns Whether the type is the api version parameter and the default value for the client
  */
 export function updateWithApiVersionInformation(
   context: TCGCContext,
   type: ModelProperty,
   client?: SdkClient | SdkOperationGroup,
+  operation?: Operation,
 ): {
   isApiVersionParam: boolean;
   clientDefaultValue?: string;
 } {
   const isApiVersionParam = isApiVersion(context, type);
-  return {
-    isApiVersionParam,
-    clientDefaultValue:
-      isApiVersionParam && client
-        ? context.__clientApiVersionDefaultValueCache.get(client)
-        : undefined,
-  };
+  if (!isApiVersionParam || !client) {
+    return { isApiVersionParam, clientDefaultValue: undefined };
+  }
+
+  // For single-service clients, use the cached value
+  if (client.services.length <= 1) {
+    return {
+      isApiVersionParam,
+      clientDefaultValue: context.__clientApiVersionDefaultValueCache.get(client),
+    };
+  }
+
+  // For multi-service clients/operation groups, we need to find the api version
+  // from the operation's specific service
+  if (operation) {
+    const service = findServiceForOperation(client.services, operation);
+    const packageVersions = context.getPackageVersions();
+    const versions = filterApiVersionsWithDecorators(
+      context,
+      type,
+      packageVersions.get(service) || [],
+    );
+    return {
+      isApiVersionParam,
+      clientDefaultValue: versions.length > 0 ? versions[versions.length - 1] : undefined,
+    };
+  }
+
+  // No operation provided for multi-service client, return undefined
+  return { isApiVersionParam, clientDefaultValue: undefined };
 }
 
 export function filterApiVersionsWithDecorators(
diff --git a/packages/typespec-client-generator-core/src/types.ts b/packages/typespec-client-generator-core/src/types.ts
index b27016db7c..40ad41ee29 100644
--- a/packages/typespec-client-generator-core/src/types.ts
+++ b/packages/typespec-client-generator-core/src/types.ts
@@ -1300,6 +1300,7 @@ export function getSdkModelPropertyTypeBase(
       context,
       type,
       operation ? context.getClientForOperation(operation) : undefined,
+      operation,
     ),
     onClient,
     crossLanguageDefinitionId: getCrossLanguageDefinitionId(context, type, operation),
diff --git a/packages/typespec-client-generator-core/test/clients/structure.test.ts b/packages/typespec-client-generator-core/test/clients/structure.test.ts
index f69d3231cf..82434ce0e9 100644
--- a/packages/typespec-client-generator-core/test/clients/structure.test.ts
+++ b/packages/typespec-client-generator-core/test/clients/structure.test.ts
@@ -1732,6 +1732,22 @@ it("client location to new operation group with multiple services", async () =>
   ok(aTestMethod);
   const bTestMethod = newOpGroup.methods.find((m) => m.name === "bTest");
   ok(bTestMethod);
+
+  // Check operation-level api version parameters have correct clientDefaultValue
+  // This is the fix for the bug - previously these were undefined
+  strictEqual(aTestMethod.kind, "basic");
+  const aOperation = aTestMethod.operation;
+  const aOperationApiVersionParam = aOperation.parameters.find((p) => p.isApiVersionParam);
+  ok(aOperationApiVersionParam);
+  // Operation from ServiceA should have ServiceA's latest api version as default
+  strictEqual(aOperationApiVersionParam.clientDefaultValue, "av2");
+
+  strictEqual(bTestMethod.kind, "basic");
+  const bOperation = bTestMethod.operation;
+  const bOperationApiVersionParam = bOperation.parameters.find((p) => p.isApiVersionParam);
+  ok(bOperationApiVersionParam);
+  // Operation from ServiceB should have ServiceB's latest api version as default
+  strictEqual(bOperationApiVersionParam.clientDefaultValue, "bv2");
 });
 
 it("one client from multiple services with operation group name conflict - merged", async () => {
@@ -1805,6 +1821,21 @@ it("one client from multiple services with operation group name conflict - merge
   ok(aTestMethod);
   const bTestMethod = operations.methods.find((m) => m.name === "bTest");
   ok(bTestMethod);
+
+  // Check operation-level api version parameters have correct clientDefaultValue
+  strictEqual(aTestMethod.kind, "basic");
+  const aOperation = aTestMethod.operation;
+  const aOperationApiVersionParam = aOperation.parameters.find((p) => p.isApiVersionParam);
+  ok(aOperationApiVersionParam);
+  // Operation from ServiceA should have ServiceA's latest api version as default
+  strictEqual(aOperationApiVersionParam.clientDefaultValue, "av2");
+
+  strictEqual(bTestMethod.kind, "basic");
+  const bOperation = bTestMethod.operation;
+  const bOperationApiVersionParam = bOperation.parameters.find((p) => p.isApiVersionParam);
+  ok(bOperationApiVersionParam);
+  // Operation from ServiceB should have ServiceB's latest api version as default
+  strictEqual(bOperationApiVersionParam.clientDefaultValue, "bv2");
 });
 
 it("client location to existing operation group from different service", async () => {