diff --git a/packages/aws-cdk-lib/aws-opensearchservice/README.md b/packages/aws-cdk-lib/aws-opensearchservice/README.md
index 236a3bf981852..e1f8477181f50 100644
--- a/packages/aws-cdk-lib/aws-opensearchservice/README.md
+++ b/packages/aws-cdk-lib/aws-opensearchservice/README.md
@@ -149,7 +149,39 @@ const domain = new Domain(this, 'Domain', {
 
 This sets up the domain with node to node encryption and encryption at
 rest. You can also choose to supply your own KMS key to use for encryption at
-rest.
+rest:
+
+```ts
+import * as kms from 'aws-cdk-lib/aws-kms';
+
+const encryptionKey = new kms.Key(this, 'EncryptionKey');
+
+const domain = new Domain(this, 'Domain', {
+  version: EngineVersion.OPENSEARCH_1_0,
+  encryptionAtRest: {
+    kmsKey: encryptionKey,
+  },
+});
+```
+
+The construct also supports using cross-account KMS keys for encryption at rest:
+
+```ts
+import * as kms from 'aws-cdk-lib/aws-kms';
+
+const crossAccountKey = kms.Key.fromKeyArn(
+  this,
+  'CrossAccountKey',
+  'arn:aws:kms:us-east-1:111111111111:key/12345678-1234-1234-1234-123456789012',
+);
+
+const domain = new Domain(this, 'Domain', {
+  version: EngineVersion.OPENSEARCH_1_0,
+  encryptionAtRest: {
+    kmsKey: crossAccountKey,
+  },
+});
+```
 
 ## VPC Support
 
diff --git a/packages/aws-cdk-lib/aws-opensearchservice/lib/domain.ts b/packages/aws-cdk-lib/aws-opensearchservice/lib/domain.ts
index a296c16785303..def3c10cfb0f4 100644
--- a/packages/aws-cdk-lib/aws-opensearchservice/lib/domain.ts
+++ b/packages/aws-cdk-lib/aws-opensearchservice/lib/domain.ts
@@ -1987,8 +1987,8 @@ export class Domain extends DomainBase implements IDomain, ec2.IConnectable {
       },
       encryptionAtRestOptions: {
         enabled: encryptionAtRestEnabled,
-        kmsKeyId: encryptionAtRestEnabled
-          ? props.encryptionAtRest?.kmsKey?.keyRef.keyId
+        kmsKeyId: encryptionAtRestEnabled && props.encryptionAtRest?.kmsKey
+          ? this.selectKmsKeyIdentifier(props.encryptionAtRest.kmsKey)
           : undefined,
       },
       nodeToNodeEncryptionOptions: { enabled: nodeToNodeEncryptionEnabled },
@@ -2197,6 +2197,42 @@ export class Domain extends DomainBase implements IDomain, ec2.IConnectable {
       }
     }
   }
+
+  /**
+   * Selects the appropriate KMS key identifier (keyId or keyArn) based on whether
+   * the key is from the same account and region as the domain.
+   *
+   * For cross-account or cross-region KMS keys, OpenSearch requires the full ARN.
+   * For same-account, same-region keys, keyId is sufficient.
+   *
+   * @param key The KMS key to use for encryption
+   * @returns The key identifier to use in CloudFormation
+   */
+  private selectKmsKeyIdentifier(key: kms.IKeyRef): string {
+    const stack = cdk.Stack.of(this);
+
+    // Check if the key is from a different account or region
+    const keyAccount = key.env.account;
+    const keyRegion = key.env.region;
+    const stackAccount = stack.account;
+    const stackRegion = stack.region;
+
+    // If either account or region is different (and not a token), use ARN
+    // Tokens are unresolved values that will be determined at deploy time
+    const isCrossAccount = !cdk.Token.isUnresolved(keyAccount) &&
+                          !cdk.Token.isUnresolved(stackAccount) &&
+                          keyAccount !== stackAccount;
+    const isCrossRegion = !cdk.Token.isUnresolved(keyRegion) &&
+                         !cdk.Token.isUnresolved(stackRegion) &&
+                         keyRegion !== stackRegion;
+
+    if (isCrossAccount || isCrossRegion) {
+      return key.keyRef.keyArn;
+    }
+
+    // For same-account, same-region keys, use keyId (maintains backward compatibility)
+    return key.keyRef.keyId;
+  }
 }
 
 /**
diff --git a/packages/aws-cdk-lib/aws-opensearchservice/test/domain.test.ts b/packages/aws-cdk-lib/aws-opensearchservice/test/domain.test.ts
index c6837c4eb3981..2b0342bb265b7 100644
--- a/packages/aws-cdk-lib/aws-opensearchservice/test/domain.test.ts
+++ b/packages/aws-cdk-lib/aws-opensearchservice/test/domain.test.ts
@@ -190,6 +190,77 @@ each([testedOpenSearchVersions]).test('grants kms permissions if needed', (engin
   expect(resources.AWS679f53fac002430cb0da5b7982bd2287ServiceRoleDefaultPolicyD28E1A5E.Properties.PolicyDocument).toStrictEqual(expectedPolicy);
 });
 
+each([testedOpenSearchVersions]).test('uses key ARN for cross-account KMS keys', (engineVersion) => {
+  // Create a cross-account KMS key using fromKeyArn
+  const crossAccountKey = kms.Key.fromKeyArn(
+    stack,
+    'CrossAccountKey',
+    'arn:aws:kms:us-east-1:999999999999:key/12345678-1234-1234-1234-123456789012',
+  );
+
+  new Domain(stack, 'Domain', {
+    version: engineVersion,
+    encryptionAtRest: {
+      kmsKey: crossAccountKey,
+    },
+  });
+
+  // Verify that the KMS key ID in the CloudFormation template is the full ARN
+  Template.fromStack(stack).hasResourceProperties('AWS::OpenSearchService::Domain', {
+    EncryptionAtRestOptions: {
+      Enabled: true,
+      KmsKeyId: 'arn:aws:kms:us-east-1:999999999999:key/12345678-1234-1234-1234-123456789012',
+    },
+  });
+});
+
+each([testedOpenSearchVersions]).test('uses key ARN for cross-region KMS keys', (engineVersion) => {
+  // Create a cross-region KMS key using fromKeyArn
+  const crossRegionKey = kms.Key.fromKeyArn(
+    stack,
+    'CrossRegionKey',
+    'arn:aws:kms:us-west-2:1234:key/12345678-1234-1234-1234-123456789012',
+  );
+
+  new Domain(stack, 'Domain', {
+    version: engineVersion,
+    encryptionAtRest: {
+      kmsKey: crossRegionKey,
+    },
+  });
+
+  // Verify that the KMS key ID in the CloudFormation template is the full ARN
+  Template.fromStack(stack).hasResourceProperties('AWS::OpenSearchService::Domain', {
+    EncryptionAtRestOptions: {
+      Enabled: true,
+      KmsKeyId: 'arn:aws:kms:us-west-2:1234:key/12345678-1234-1234-1234-123456789012',
+    },
+  });
+});
+
+each([testedOpenSearchVersions]).test('uses key ID for same-account same-region KMS keys', (engineVersion) => {
+  // Create a same-account, same-region KMS key
+  const key = new kms.Key(stack, 'Key');
+
+  new Domain(stack, 'Domain', {
+    version: engineVersion,
+    encryptionAtRest: {
+      kmsKey: key,
+    },
+  });
+
+  // Verify that the KMS key ID in the CloudFormation template uses keyId (not ARN)
+  // This maintains backward compatibility - keyId returns a Ref to the key
+  Template.fromStack(stack).hasResourceProperties('AWS::OpenSearchService::Domain', {
+    EncryptionAtRestOptions: {
+      Enabled: true,
+      KmsKeyId: {
+        Ref: 'Key961B73FD',
+      },
+    },
+  });
+});
+
 each([
   [EngineVersion.OPENSEARCH_1_0, 'OpenSearch_1.0'],
   [EngineVersion.OPENSEARCH_1_1, 'OpenSearch_1.1'],