diff --git a/packages/aws-cdk-lib/aws-elasticloadbalancingv2/lib/alb/application-target-group.ts b/packages/aws-cdk-lib/aws-elasticloadbalancingv2/lib/alb/application-target-group.ts
index f5c181ad3ddce..111170034b4f6 100644
--- a/packages/aws-cdk-lib/aws-elasticloadbalancingv2/lib/alb/application-target-group.ts
+++ b/packages/aws-cdk-lib/aws-elasticloadbalancingv2/lib/alb/application-target-group.ts
@@ -404,6 +404,14 @@ export class ApplicationTargetGroup extends TargetGroupBase implements IApplicat
         this.setAttribute('load_balancing.algorithm.anomaly_mitigation', props.enableAnomalyMitigation ? 'on' : 'off');
       }
     }
+
+    // Add a warning if neither protocol nor port was explicitly specified for non-Lambda targets
+    if (props.protocol === undefined && props.port === undefined && this.targetType !== TargetType.LAMBDA) {
+      Annotations.of(this).addWarningV2(
+        '@aws-cdk/aws-elbv2:target-group-protocol-default',
+        'ApplicationTargetGroup protocol and port not specified. Please specify either protocol (e.g., ApplicationProtocol.HTTP or ApplicationProtocol.HTTPS) or port number. Note: Missing both will cause deployment failures.',
+      );
+    }
   }
 
   public get metrics(): IApplicationTargetGroupMetrics {
@@ -614,10 +622,8 @@ export class ApplicationTargetGroup extends TargetGroupBase implements IApplicat
   protected validateTargetGroup(): string[] {
     const ret = super.validateTargetGroup();
 
-    if (this.targetType !== undefined && this.targetType !== TargetType.LAMBDA
-      && (this.protocol === undefined || this.port === undefined)) {
-      ret.push('At least one of \'port\' or \'protocol\' is required for a non-Lambda TargetGroup');
-    }
+    // Note: Protocol/port validation is now handled as a warning in the constructor
+    // instead of a hard validation error to maintain backward compatibility
 
     if (this.healthCheck) {
       if (this.healthCheck.interval && this.healthCheck.timeout &&
diff --git a/packages/aws-cdk-lib/aws-elasticloadbalancingv2/test/alb/target-group.test.ts b/packages/aws-cdk-lib/aws-elasticloadbalancingv2/test/alb/target-group.test.ts
index c15a13b77479f..893c9305dee5d 100644
--- a/packages/aws-cdk-lib/aws-elasticloadbalancingv2/test/alb/target-group.test.ts
+++ b/packages/aws-cdk-lib/aws-elasticloadbalancingv2/test/alb/target-group.test.ts
@@ -71,6 +71,91 @@ describe('tests', () => {
     expect(() => app.synth()).toThrow(/port\/protocol should not be specified for Lambda targets/);
   });
 
+  test('ApplicationTargetGroup shows warning when neither protocol nor port specified', () => {
+    // GIVEN
+    const vpc = new ec2.Vpc(stack, 'Vpc');
+
+    // WHEN - Create ApplicationTargetGroup without protocol or port
+    new elbv2.ApplicationTargetGroup(stack, 'TG', {
+      vpc,
+      // Neither protocol nor port specified - this should generate a warning
+    });
+
+    // THEN - Should synthesize successfully but show warning
+    const assembly = app.synth();
+    const stackArtifact = assembly.getStackByName(stack.stackName);
+    const warnings = stackArtifact.messages.filter(m => m.level === 'warning');
+    expect(warnings.some(w => w.entry.data && typeof w.entry.data === 'string' &&
+      w.entry.data.includes('ApplicationTargetGroup protocol and port not specified'))).toBe(true);
+  });
+
+  test('ApplicationTargetGroup with port but no protocol should not show warning', () => {
+    // GIVEN
+    const vpc = new ec2.Vpc(stack, 'Vpc');
+
+    // WHEN - Create ApplicationTargetGroup without protocol but with port
+    new elbv2.ApplicationTargetGroup(stack, 'TG', {
+      vpc,
+      port: 80, // Providing port satisfies the requirement - protocol will be determined automatically
+    });
+
+    // THEN - Should synthesize successfully without warning
+    const assembly = app.synth();
+    const stackArtifact = assembly.getStackByName(stack.stackName);
+    const warnings = stackArtifact.messages.filter(m => m.level === 'warning');
+    expect(warnings.some(w => w.entry.data && typeof w.entry.data === 'string' &&
+      w.entry.data.includes('ApplicationTargetGroup protocol and port not specified'))).toBe(false);
+  });
+
+  test('ApplicationTargetGroup with protocol should not throw validation error', () => {
+    // GIVEN
+    const vpc = new ec2.Vpc(stack, 'Vpc');
+
+    // WHEN - Create ApplicationTargetGroup with protocol
+    new elbv2.ApplicationTargetGroup(stack, 'TG', {
+      vpc,
+      port: 80,
+      protocol: elbv2.ApplicationProtocol.HTTP,
+    });
+
+    // THEN - Should not throw
+    expect(() => app.synth()).not.toThrow();
+  });
+
+  test('ApplicationTargetGroup with Lambda target does not require protocol', () => {
+    // GIVEN
+
+    // WHEN - Create ApplicationTargetGroup for Lambda without protocol
+    new elbv2.ApplicationTargetGroup(stack, 'TG', {
+      targetType: elbv2.TargetType.LAMBDA,
+      // protocol is missing but should be allowed for Lambda
+    });
+
+    // THEN - Should not throw
+    expect(() => app.synth()).not.toThrow();
+  });
+
+  test('ApplicationTargetGroup requires protocol or port even when targets added later', () => {
+    // GIVEN
+    const vpc = new ec2.Vpc(stack, 'Vpc');
+
+    // WHEN - Create ApplicationTargetGroup without protocol or port, then add non-Lambda target
+    const tg = new elbv2.ApplicationTargetGroup(stack, 'TG', {
+      vpc,
+      // Neither protocol nor port specified - this should generate a warning
+    });
+
+    // Add a non-Lambda target
+    tg.addTarget(new elbv2.InstanceTarget('i-1234'));
+
+    // THEN - Should synthesize successfully but show warning
+    const assembly = app.synth();
+    const stackArtifact = assembly.getStackByName(stack.stackName);
+    const warnings = stackArtifact.messages.filter(m => m.level === 'warning');
+    expect(warnings.some(w => w.entry.data && typeof w.entry.data === 'string' &&
+      w.entry.data.includes('ApplicationTargetGroup protocol and port not specified'))).toBe(true);
+  });
+
   test('Can add self-registering target to imported TargetGroup', () => {
     // GIVEN
     const vpc = new ec2.Vpc(stack, 'Vpc');