diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index b7dd84f7716..db0e4e1575d 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -23008,6 +23008,13 @@ namespace ts {
                 !(getModifierFlags(parameter) & ModifierFlags.ParameterPropertyModifier);
         }
 
+        function isOptionalUninitializedParameterProperty(parameter: ParameterDeclaration) {
+            return strictNullChecks &&
+                isOptionalParameter(parameter) &&
+                !parameter.initializer &&
+                !!(getModifierFlags(parameter) & ModifierFlags.ParameterPropertyModifier);
+        }
+
         function getNodeCheckFlags(node: Node): NodeCheckFlags {
             return getNodeLinks(node).flags;
         }
@@ -23217,6 +23224,7 @@ namespace ts {
                 isDeclarationVisible,
                 isImplementationOfOverload,
                 isRequiredInitializedParameter,
+                isOptionalUninitializedParameterProperty,
                 writeTypeOfDeclaration,
                 writeReturnTypeOfSignatureDeclaration,
                 writeTypeOfExpression,
diff --git a/src/compiler/declarationEmitter.ts b/src/compiler/declarationEmitter.ts
index 007620ad510..753080cc6a0 100644
--- a/src/compiler/declarationEmitter.ts
+++ b/src/compiler/declarationEmitter.ts
@@ -335,9 +335,12 @@ namespace ts {
             write(": ");
 
             // use the checker's type, not the declared type,
-            // for non-optional initialized parameters that aren't a parameter property
+            // for optional parameter properties
+            // and also for non-optional initialized parameters that aren't a parameter property
+            // these types may need to add `undefined`.
             const shouldUseResolverType = declaration.kind === SyntaxKind.Parameter &&
-                resolver.isRequiredInitializedParameter(declaration as ParameterDeclaration);
+                (resolver.isRequiredInitializedParameter(declaration as ParameterDeclaration) ||
+                 resolver.isOptionalUninitializedParameterProperty(declaration as ParameterDeclaration));
             if (type && !shouldUseResolverType) {
                 // Write the type
                 emitType(type);
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 75eabac49b0..76be7b20b1d 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2814,6 +2814,7 @@ namespace ts {
         collectLinkedAliases(node: Identifier): Node[];
         isImplementationOfOverload(node: FunctionLikeDeclaration): boolean | undefined;
         isRequiredInitializedParameter(node: ParameterDeclaration): boolean;
+        isOptionalUninitializedParameterProperty(node: ParameterDeclaration): boolean;
         writeTypeOfDeclaration(declaration: AccessorDeclaration | VariableLikeDeclaration, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter): void;
         writeReturnTypeOfSignatureDeclaration(signatureDeclaration: SignatureDeclaration, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter): void;
         writeTypeOfExpression(expr: Expression, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter): void;
diff --git a/tests/baselines/reference/declarationEmitParameterProperty.js b/tests/baselines/reference/declarationEmitParameterProperty.js
new file mode 100644
index 00000000000..262222e3c68
--- /dev/null
+++ b/tests/baselines/reference/declarationEmitParameterProperty.js
@@ -0,0 +1,24 @@
+//// [declarationEmitParameterProperty.ts]
+export class Foo {
+  constructor(public bar?: string) {
+  }
+}
+
+
+//// [declarationEmitParameterProperty.js]
+"use strict";
+exports.__esModule = true;
+var Foo = (function () {
+    function Foo(bar) {
+        this.bar = bar;
+    }
+    return Foo;
+}());
+exports.Foo = Foo;
+
+
+//// [declarationEmitParameterProperty.d.ts]
+export declare class Foo {
+    bar: string | undefined;
+    constructor(bar?: string | undefined);
+}
diff --git a/tests/baselines/reference/declarationEmitParameterProperty.symbols b/tests/baselines/reference/declarationEmitParameterProperty.symbols
new file mode 100644
index 00000000000..d9e88e52cb0
--- /dev/null
+++ b/tests/baselines/reference/declarationEmitParameterProperty.symbols
@@ -0,0 +1,9 @@
+=== tests/cases/compiler/declarationEmitParameterProperty.ts ===
+export class Foo {
+>Foo : Symbol(Foo, Decl(declarationEmitParameterProperty.ts, 0, 0))
+
+  constructor(public bar?: string) {
+>bar : Symbol(Foo.bar, Decl(declarationEmitParameterProperty.ts, 1, 14))
+  }
+}
+
diff --git a/tests/baselines/reference/declarationEmitParameterProperty.types b/tests/baselines/reference/declarationEmitParameterProperty.types
new file mode 100644
index 00000000000..3adfecf0c75
--- /dev/null
+++ b/tests/baselines/reference/declarationEmitParameterProperty.types
@@ -0,0 +1,9 @@
+=== tests/cases/compiler/declarationEmitParameterProperty.ts ===
+export class Foo {
+>Foo : Foo
+
+  constructor(public bar?: string) {
+>bar : string | undefined
+  }
+}
+
diff --git a/tests/baselines/reference/optionalMethods.js b/tests/baselines/reference/optionalMethods.js
index d219466c562..2abcd209277 100644
--- a/tests/baselines/reference/optionalMethods.js
+++ b/tests/baselines/reference/optionalMethods.js
@@ -131,12 +131,12 @@ interface Foo {
 }
 declare function test1(x: Foo): void;
 declare class Bar {
-    d: number;
+    d: number | undefined;
     e: number;
     a: number;
     b?: number;
     c?: number | undefined;
-    constructor(d?: number, e?: number);
+    constructor(d?: number | undefined, e?: number);
     f(): number;
     g?(): number;
     h?(): number;
diff --git a/tests/cases/compiler/declarationEmitParameterProperty.ts b/tests/cases/compiler/declarationEmitParameterProperty.ts
new file mode 100644
index 00000000000..107d192fe88
--- /dev/null
+++ b/tests/cases/compiler/declarationEmitParameterProperty.ts
@@ -0,0 +1,6 @@
+// @strictNullChecks: true
+// @declaration: true
+export class Foo {
+  constructor(public bar?: string) {
+  }
+}