diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
index 992f8596d08fe..3372512e0bca1 100644
--- a/src/compiler/binder.ts
+++ b/src/compiler/binder.ts
@@ -3595,7 +3595,8 @@ namespace ts {
             || hasModifier(node, ModifierFlags.TypeScriptModifier)
             || node.typeParameters
             || node.type
-            || !node.body) {
+            || !node.body
+            || node.questionToken) {
             transformFlags |= TransformFlags.AssertTypeScript;
         }
 
@@ -3642,7 +3643,7 @@ namespace ts {
         let transformFlags = subtreeFlags | TransformFlags.ContainsClassFields;
 
         // Decorators, TypeScript-specific modifiers, and type annotations are TypeScript syntax.
-        if (some(node.decorators) || hasModifier(node, ModifierFlags.TypeScriptModifier) || node.type) {
+        if (some(node.decorators) || hasModifier(node, ModifierFlags.TypeScriptModifier) || node.type || node.questionToken) {
             transformFlags |= TransformFlags.AssertTypeScript;
         }
 
diff --git a/tests/baselines/reference/optionalMethod.js b/tests/baselines/reference/optionalMethod.js
new file mode 100644
index 0000000000000..41680ce7261f9
--- /dev/null
+++ b/tests/baselines/reference/optionalMethod.js
@@ -0,0 +1,10 @@
+//// [optionalMethod.ts]
+class Base {
+    method?() { }
+}
+
+
+//// [optionalMethod.js]
+class Base {
+    method() { }
+}
diff --git a/tests/baselines/reference/optionalProperty.js b/tests/baselines/reference/optionalProperty.js
new file mode 100644
index 0000000000000..d7c37f64914ee
--- /dev/null
+++ b/tests/baselines/reference/optionalProperty.js
@@ -0,0 +1,10 @@
+//// [optionalProperty.ts]
+class C {
+    prop?;
+}
+
+
+//// [optionalProperty.js]
+class C {
+    prop;
+}
diff --git a/tests/cases/conformance/classes/propertyMemberDeclarations/optionalMethod.ts b/tests/cases/conformance/classes/propertyMemberDeclarations/optionalMethod.ts
new file mode 100644
index 0000000000000..39ea6df3bd5dc
--- /dev/null
+++ b/tests/cases/conformance/classes/propertyMemberDeclarations/optionalMethod.ts
@@ -0,0 +1,5 @@
+// @target: esnext
+// @noTypesAndSymbols: true
+class Base {
+    method?() { }
+}
diff --git a/tests/cases/conformance/classes/propertyMemberDeclarations/optionalProperty.ts b/tests/cases/conformance/classes/propertyMemberDeclarations/optionalProperty.ts
new file mode 100644
index 0000000000000..cb563eedf1867
--- /dev/null
+++ b/tests/cases/conformance/classes/propertyMemberDeclarations/optionalProperty.ts
@@ -0,0 +1,6 @@
+// @target: esnext
+// @useDefineForClassFields: true
+// @noTypesAndSymbols: true
+class C {
+    prop?;
+}