diff --git a/handwritten/storage/src/bucket.ts b/handwritten/storage/src/bucket.ts
index fde1a9bfd18..f57a213df95 100644
--- a/handwritten/storage/src/bucket.ts
+++ b/handwritten/storage/src/bucket.ts
@@ -297,6 +297,10 @@ export interface RestoreOptions {
   generation: string;
   projection?: 'full' | 'noAcl';
 }
+export interface EncryptionEnforcementConfig {
+  restrictionMode?: 'NotRestricted' | 'FullyRestricted';
+  readonly effectiveTime?: string;
+}
 export interface BucketMetadata extends BaseMetadata {
   acl?: AclMetadata[] | null;
   autoclass?: {
@@ -316,6 +320,9 @@ export interface BucketMetadata extends BaseMetadata {
   defaultObjectAcl?: AclMetadata[];
   encryption?: {
     defaultKmsKeyName?: string;
+    googleManagedEncryptionEnforcementConfig?: EncryptionEnforcementConfig;
+    customerManagedEncryptionEnforcementConfig?: EncryptionEnforcementConfig;
+    customerSuppliedEncryptionEnforcementConfig?: EncryptionEnforcementConfig;
   } | null;
   hierarchicalNamespace?: {
     enabled?: boolean;
@@ -1193,6 +1200,25 @@ class Bucket extends ServiceObject<Bucket, BucketMetadata> {
        * }, function(err, apiResponse) {});
        *
        * //-
+       * // Enforce CMEK-only encryption for new objects.
+       * // This blocks Google-Managed and Customer-Supplied keys.
+       * //-
+       * bucket.setMetadata({
+       *   encryption: {
+       *     defaultKmsKeyName: 'projects/grape-spaceship-123/...',
+       *     googleManagedEncryptionEnforcementConfig: {
+       *       restrictionMode: 'FullyRestricted'
+       *     },
+       *     customerSuppliedEncryptionEnforcementConfig: {
+       *       restrictionMode: 'FullyRestricted'
+       *     },
+       *     customerManagedEncryptionEnforcementConfig: {
+       *       restrictionMode: 'NotRestricted'
+       *     }
+       *   }
+       * }, function(err, apiResponse) {});
+       *
+       * //-
        * // Set the default event-based hold value for new objects in this
        * // bucket.
        * //-
diff --git a/handwritten/storage/system-test/storage.ts b/handwritten/storage/system-test/storage.ts
index 2f14fe01296..3edee247249 100644
--- a/handwritten/storage/system-test/storage.ts
+++ b/handwritten/storage/system-test/storage.ts
@@ -2992,6 +2992,89 @@ describe('storage', function () {
             `${metadata!.encryption!.defaultKmsKeyName}/cryptoKeyVersions/1`,
           );
         });
+
+        describe('encryption enforcement', () => {
+          it('should enforce FullyRestricted CSEK policy', async () => {
+            await bucket.setMetadata({
+              encryption: {
+                defaultKmsKeyName: kmsKeyName,
+                customerSuppliedEncryptionEnforcementConfig: {
+                  restrictionMode: 'FullyRestricted',
+                },
+              },
+            });
+
+            await new Promise(res =>
+              setTimeout(res, BUCKET_METADATA_UPDATE_WAIT_TIME)
+            );
+
+            const encryptionKey = crypto.randomBytes(32);
+            const file = bucket.file('csek-attempt', {encryptionKey});
+
+            await assert.rejects(
+              file.save(FILE_CONTENTS, {resumable: false}),
+              (err: ApiError) => {
+                const failureMessage =
+                  "Requested encryption type for object is not compliant with the bucket's encryption enforcement configuration.";
+                assert.strictEqual(err.code, 412);
+                assert.ok(err.message.includes(failureMessage));
+                return true;
+              }
+            );
+          });
+
+          it('should allow uploads that comply with enforcement', async () => {
+            await bucket.setMetadata({
+              encryption: {
+                googleManagedEncryptionEnforcementConfig: {
+                  restrictionMode: 'NotRestricted',
+                },
+              },
+            });
+
+            const file = bucket.file('compliant-file');
+            await file.save(FILE_CONTENTS);
+
+            const [metadata] = await file.getMetadata();
+            assert.ok(metadata.customerEncryption);
+          });
+
+          it('should retain defaultKmsKeyName when updating enforcement settings independently', async () => {
+            await bucket.setMetadata({
+              encryption: {
+                defaultKmsKeyName: kmsKeyName,
+              },
+            });
+
+            await new Promise(res =>
+              setTimeout(res, BUCKET_METADATA_UPDATE_WAIT_TIME)
+            );
+
+            await bucket.setMetadata({
+              encryption: {
+                googleManagedEncryptionEnforcementConfig: {
+                  restrictionMode: 'FullyRestricted',
+                },
+              },
+            });
+
+            await new Promise(res =>
+              setTimeout(res, BUCKET_METADATA_UPDATE_WAIT_TIME)
+            );
+
+            const [metadata] = await bucket.getMetadata();
+            assert.strictEqual(
+              metadata.encryption?.defaultKmsKeyName,
+              kmsKeyName
+            );
+
+            assert.strictEqual(
+              metadata.encryption?.googleManagedEncryptionEnforcementConfig
+                ?.restrictionMode,
+              'FullyRestricted'
+            );
+          });
+        });
       });
     });
 
diff --git a/handwritten/storage/test/bucket.ts b/handwritten/storage/test/bucket.ts
index 5b49fa518d8..f59f41e21a1 100644
--- a/handwritten/storage/test/bucket.ts
+++ b/handwritten/storage/test/bucket.ts
@@ -3300,4 +3300,145 @@ describe('Bucket', () => {
       done();
     });
   });
+
+  describe('setMetadata', () => {
+    describe('encryption enforcement', () => {
+      it('should correctly format restrictionMode for all enforcement types', () => {
+        const effectiveTime = '2026-02-02T12:00:00Z';
+        const encryptionMetadata = {
+          encryption: {
+            defaultKmsKeyName: 'kms-key-name',
+            googleManagedEncryptionEnforcementConfig: {
+              restrictionMode: 'FullyRestricted',
+              effectiveTime: effectiveTime,
+            },
+            customerManagedEncryptionEnforcementConfig: {
+              restrictionMode: 'NotRestricted',
+              effectiveTime: effectiveTime,
+            },
+            customerSuppliedEncryptionEnforcementConfig: {
+              restrictionMode: 'FullyRestricted',
+              effectiveTime: effectiveTime,
+            },
+          },
+        };
+
+        bucket.setMetadata = (metadata: BucketMetadata) => {
+          assert.strictEqual(
+            metadata.encryption?.defaultKmsKeyName,
+            encryptionMetadata.encryption.defaultKmsKeyName
+          );
+
+          assert.deepStrictEqual(
+            metadata.encryption?.googleManagedEncryptionEnforcementConfig,
+            {restrictionMode: 'FullyRestricted', effectiveTime: effectiveTime}
+          );
+
+          assert.deepStrictEqual(
+            metadata.encryption?.customerManagedEncryptionEnforcementConfig,
+            {restrictionMode: 'NotRestricted', effectiveTime: effectiveTime}
+          );
+
+          assert.deepStrictEqual(
+            metadata.encryption?.customerSuppliedEncryptionEnforcementConfig,
+            {restrictionMode: 'FullyRestricted', effectiveTime: effectiveTime}
+          );
+        };
+        bucket.setMetadata(encryptionMetadata, assert.ifError);
+      });
+
+      it('should preserve existing encryption fields during a partial update', done => {
+        bucket.metadata = {
+          encryption: {
+            defaultKmsKeyName: 'kms-key-name',
+            googleManagedEncryptionEnforcementConfig: {
+              restrictionMode: 'FullyRestricted',
+            },
+          },
+        };
+
+        const patch = {
+          encryption: {
+            customerSuppliedEncryptionEnforcementConfig: {
+              restrictionMode: 'FullyRestricted',
+            },
+          },
+        };
+
+        bucket.setMetadata = (metadata: BucketMetadata) => {
+          assert.strictEqual(
+            metadata.encryption?.customerSuppliedEncryptionEnforcementConfig
+              ?.restrictionMode,
+            'FullyRestricted'
+          );
+          done();
+        };
+
+        bucket.setMetadata(patch, assert.ifError);
+      });
+
+      it('should reject or handle invalid restrictionMode values', done => {
+        const invalidMetadata = {
+          encryption: {
+            googleManagedEncryptionEnforcementConfig: {
+              restrictionMode: 'fully_restricted',
+            },
+          },
+        };
+
+        bucket.setMetadata = (metadata: BucketMetadata) => {
+          assert.strictEqual(
+            metadata.encryption?.googleManagedEncryptionEnforcementConfig
+              ?.restrictionMode,
+            'fully_restricted'
+          );
+          done();
+        };
+
+        bucket.setMetadata(invalidMetadata, assert.ifError);
+      });
+
+      it('should not include enforcement configs that are not provided', done => {
+        const partialMetadata = {
+          encryption: {
+            defaultKmsKeyName: 'test-key',
+            googleManagedEncryptionEnforcementConfig: {
+              restrictionMode: 'FullyRestricted',
+            },
+          },
+        };
+
+        bucket.setMetadata = (metadata: BucketMetadata) => {
+          assert.ok(metadata.encryption?.defaultKmsKeyName);
+          assert.ok(
+            metadata.encryption?.googleManagedEncryptionEnforcementConfig
+          );
+          assert.strictEqual(
+            metadata.encryption?.customerManagedEncryptionEnforcementConfig,
+            undefined
+          );
+          assert.strictEqual(
+            metadata.encryption?.customerSuppliedEncryptionEnforcementConfig,
+            undefined
+          );
+          done();
+        };
+
+        bucket.setMetadata(partialMetadata, assert.ifError);
+      });
+
+      it('should allow nullifying encryption enforcement', done => {
+        const clearMetadata = {
+          encryption: null,
+        };
+
+        bucket.setMetadata = (metadata: BucketMetadata) => {
+          assert.strictEqual(metadata.encryption, null);
+          done();
+        };
+
+        bucket.setMetadata(clearMetadata, assert.ifError);
+      });
+    });
+  });
 });