diff --git a/.env.example b/.env.example
index 01f153f..5869f05 100644
--- a/.env.example
+++ b/.env.example
@@ -22,6 +22,10 @@ MESH_MEMBER_HEARTBEAT_TTL_SECONDS=600
 # Maximum connection time (expiresAt) for a group
 MESH_MAX_CONNECTION_TIME_SECONDS=1500
 
+# Custom Domain Settings
+# APPSYNC_CUSTOM_DOMAIN=graphql.api.smalruby.app
+# ROUTE53_PARENT_ZONE_NAME=api.smalruby.app
+
 # Development Recommended Values (uncomment to use)
 # MESH_HOST_HEARTBEAT_INTERVAL_SECONDS=15
 # MESH_HOST_HEARTBEAT_TTL_SECONDS=60
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e51e476..9dd035f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -49,6 +49,8 @@ jobs:
 
       - name: Synthesize CDK stack
         run: npx cdk synth --context stage=stg
+        env:
+          APPSYNC_CUSTOM_DOMAIN: 'false'
 
   security:
     name: Security Audits
diff --git a/bin/mesh-v2.ts b/bin/mesh-v2.ts
index a7f94f2..31d450b 100644
--- a/bin/mesh-v2.ts
+++ b/bin/mesh-v2.ts
@@ -15,17 +15,8 @@ const stackName = stage === 'prod' ? 'MeshV2Stack' : `MeshV2Stack-${stage}`;
 
 new MeshV2Stack(app, stackName, {
   stackName: stackName,
-  /* If you don't specify 'env', this stack will be environment-agnostic.
-   * Account/Region-dependent features and context lookups will not work,
-   * but a single synthesized template can be deployed anywhere. */
-
-  /* Uncomment the next line to specialize this stack for the AWS Account
-   * and Region that are implied by the current CLI configuration. */
-  // env: { account: process.env.CDK_DEFAULT_ACCOUNT, region: process.env.CDK_DEFAULT_REGION },
-
-  /* Uncomment the next line if you know exactly what Account and Region you
-   * want to deploy the stack to. */
-  // env: { account: '123456789012', region: 'us-east-1' },
-
-  /* For more information, see https://docs.aws.amazon.com/cdk/latest/guide/environments.html */
+  env: {
+    account: process.env.CDK_DEFAULT_ACCOUNT || process.env.AWS_ACCOUNT_ID,
+    region: process.env.CDK_DEFAULT_REGION || process.env.AWS_REGION || 'ap-northeast-1',
+  },
 });
diff --git a/cdk.context.json b/cdk.context.json
index e26be24..4be60ea 100644
--- a/cdk.context.json
+++ b/cdk.context.json
@@ -1,5 +1,9 @@
 {
   "acknowledged-issue-numbers": [
     34892
-  ]
+  ],
+  "hosted-zone:account=007325983811:domainName=api.smalruby.app:region=ap-northeast-1": {
+    "Id": "/hostedzone/Z04680371PP076PW332Q5",
+    "Name": "api.smalruby.app."
+  }
 }
diff --git a/lib/mesh-v2-stack.ts b/lib/mesh-v2-stack.ts
index 8662d4f..15e7c6a 100644
--- a/lib/mesh-v2-stack.ts
+++ b/lib/mesh-v2-stack.ts
@@ -2,6 +2,9 @@ import * as cdk from 'aws-cdk-lib/core';
 import * as dynamodb from 'aws-cdk-lib/aws-dynamodb';
 import * as appsync from 'aws-cdk-lib/aws-appsync';
 import * as lambda from 'aws-cdk-lib/aws-lambda';
+import * as route53 from 'aws-cdk-lib/aws-route53';
+import * as acm from 'aws-cdk-lib/aws-certificatemanager';
+import * as targets from 'aws-cdk-lib/aws-route53-targets';
 import * as path from 'path';
 import { Construct } from 'constructs';
 
@@ -16,6 +19,33 @@ export class MeshV2Stack extends cdk.Stack {
     const stage = this.node.tryGetContext('stage') || process.env.STAGE || 'stg';
     const stageSuffix = stage === 'prod' ? '' : `-${stage}`;
 
+    // Custom Domain configuration from environment variables
+    const parentZoneName = process.env.ROUTE53_PARENT_ZONE_NAME || 'api.smalruby.app';
+    const defaultCustomDomain = stage === 'prod' ? `graphql.${parentZoneName}` : `${stage}.graphql.${parentZoneName}`;
+    const customDomain = process.env.APPSYNC_CUSTOM_DOMAIN === 'false'
+      ? undefined
+      : (process.env.APPSYNC_CUSTOM_DOMAIN || defaultCustomDomain);
+
+    let domainOptions: appsync.DomainOptions | undefined;
+    let zone: route53.IHostedZone | undefined;
+
+    if (customDomain) {
+      zone = route53.HostedZone.fromLookup(this, 'HostedZone', {
+        domainName: parentZoneName,
+      });
+
+      const certificate = new acm.DnsValidatedCertificate(this, 'ApiCertificate', {
+        domainName: customDomain,
+        hostedZone: zone,
+        region: 'us-east-1',
+      });
+
+      domainOptions = {
+        certificate,
+        domainName: customDomain,
+      };
+    }
+
     // Stack全体にタグ付与
     cdk.Tags.of(this).add('Project', 'MeshV2');
     cdk.Tags.of(this).add('Stage', stage);
@@ -77,6 +107,7 @@ export class MeshV2Stack extends cdk.Stack {
     this.api = new appsync.GraphqlApi(this, 'MeshV2Api', {
       name: `MeshV2Api${stageSuffix}`,
       definition: appsync.Definition.fromFile(path.join(__dirname, '../graphql/schema.graphql')),
+      domainName: domainOptions,
       authorizationConfig: {
         defaultAuthorization: {
           authorizationType: appsync.AuthorizationType.API_KEY,
@@ -100,6 +131,18 @@ export class MeshV2Stack extends cdk.Stack {
       },
     });
 
+    // Route53 Alias record for Custom Domain
+    if (customDomain && zone) {
+      // Extract subdomain from customDomain (e.g., "graphql.api.smalruby.app" -> "graphql")
+      const subdomain = customDomain.replace(`.${parentZoneName}`, '');
+
+      new route53.ARecord(this, 'ApiAliasRecord', {
+        zone,
+        recordName: subdomain,
+        target: route53.RecordTarget.fromAlias(new targets.AppSyncTarget(this.api)),
+      });
+    }
+
     // AppSync APIにタグ付与
     cdk.Tags.of(this.api).add('ResourceType', 'GraphQLAPI');
 
@@ -420,5 +463,13 @@ export class MeshV2Stack extends cdk.Stack {
       value: this.api.apiId,
       description: 'AppSync GraphQL API ID',
     });
+
+    // Output Custom Domain URL
+    if (customDomain) {
+      new cdk.CfnOutput(this, 'CustomDomainUrl', {
+        value: `https://${customDomain}/graphql`,
+        description: 'AppSync Custom Domain URL',
+      });
+    }
   }
 }
diff --git a/spec/requests/subscriptions_spec.rb b/spec/requests/subscriptions_spec.rb
index aea171b..7deb3c1 100644
--- a/spec/requests/subscriptions_spec.rb
+++ b/spec/requests/subscriptions_spec.rb
@@ -138,7 +138,7 @@
       puts "2. Connect to AppSync WebSocket endpoint:"
       puts "   API_URL='#{ENV["APPSYNC_ENDPOINT"]}'"
       puts "   API_KEY='#{ENV["APPSYNC_API_KEY"]}'"
-      puts "   WS_URL=$(echo $API_URL | sed 's/https:/wss:/g' | sed 's/graphql$/graphql\\/connect/g')"
+      puts "   WS_URL=$(echo $API_URL | sed 's/https:/wss:/g' | sed 's/graphql$/graphql\\/realtime/g')"
       puts ""
       puts "3. Subscribe to onMessageInGroup:"
       puts "   subscription {"
diff --git a/spec/support/appsync_subscription_helper.rb b/spec/support/appsync_subscription_helper.rb
index 9d3f43a..f1f6986 100644
--- a/spec/support/appsync_subscription_helper.rb
+++ b/spec/support/appsync_subscription_helper.rb
@@ -132,9 +132,16 @@ def subscribe_and_execute(query, variables = {}, wait_time: 2, timeout: 15, &blo
   private
 
   def convert_to_websocket_endpoint(https_endpoint)
-    https_endpoint
-      .sub("https://", "wss://")
-      .sub(".appsync-api.", ".appsync-realtime-api.")
+    if https_endpoint.include?(".appsync-api.")
+      https_endpoint
+        .sub("https://", "wss://")
+        .sub(".appsync-api.", ".appsync-realtime-api.")
+    else
+      # For custom domains, AppSync WebSocket endpoint requires /realtime path
+      https_endpoint
+        .sub("https://", "wss://")
+        .sub(/\/graphql\/?$/, "/graphql/realtime")
+    end
   end
 
   def extract_host(endpoint)