diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index a86f18e4cceac..3eb8267de7999 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4165,7 +4165,17 @@ namespace ts {
                 return getTypeForBindingElement(<BindingElement>declaration);
             }
 
-            const isOptional = !isBindingElement(declaration) && !isVariableDeclaration(declaration) && !!declaration.questionToken && includeOptionality;
+            let isOptional = false;
+            if (includeOptionality) {
+                if (isInJavaScriptFile(declaration) && isParameter(declaration)) {
+                    const parameterTags = getJSDocParameterTags(declaration);
+                    isOptional = !!(parameterTags && parameterTags.length > 0 && find(parameterTags, tag => tag.isBracketed));
+                }
+                if (!isBindingElement(declaration) && !isVariableDeclaration(declaration) && !!declaration.questionToken) {
+                    isOptional = true;
+                }
+            }
+
             // Use type from type annotation if one is present
             const declaredType = tryGetTypeFromEffectiveTypeNode(declaration);
             if (declaredType) {
@@ -8668,9 +8678,10 @@ namespace ts {
                     return getTypeFromIntersectionTypeNode(<IntersectionTypeNode>node);
                 case SyntaxKind.JSDocNullableType:
                     return getTypeFromJSDocNullableTypeNode(<JSDocNullableType>node);
+                case SyntaxKind.JSDocOptionalType:
+                    return addOptionality(getTypeFromTypeNode((node as JSDocOptionalType).type));
                 case SyntaxKind.ParenthesizedType:
                 case SyntaxKind.JSDocNonNullableType:
-                case SyntaxKind.JSDocOptionalType:
                 case SyntaxKind.JSDocTypeExpression:
                     return getTypeFromTypeNode((<ParenthesizedTypeNode | JSDocTypeReferencingNode | JSDocTypeExpression>node).type);
                 case SyntaxKind.JSDocVariadicType:
diff --git a/tests/baselines/reference/jsdocParamTagTypeLiteral.types b/tests/baselines/reference/jsdocParamTagTypeLiteral.types
index c705647c9d2f5..35b472e81bec5 100644
--- a/tests/baselines/reference/jsdocParamTagTypeLiteral.types
+++ b/tests/baselines/reference/jsdocParamTagTypeLiteral.types
@@ -23,18 +23,18 @@ normal(12);
  * @param {string} [opts1.w="hi"] doc5
  */
 function foo1(opts1) {
->foo1 : (opts1: { x: string; y?: string; z?: string; w?: string; }) => void
->opts1 : { x: string; y?: string; z?: string; w?: string; }
+>foo1 : (opts1: { x: string; y?: string | undefined; z?: string; w?: string; }) => void
+>opts1 : { x: string; y?: string | undefined; z?: string; w?: string; }
 
     opts1.x;
 >opts1.x : string
->opts1 : { x: string; y?: string; z?: string; w?: string; }
+>opts1 : { x: string; y?: string | undefined; z?: string; w?: string; }
 >x : string
 }
 
 foo1({x: 'abc'});
 >foo1({x: 'abc'}) : void
->foo1 : (opts1: { x: string; y?: string; z?: string; w?: string; }) => void
+>foo1 : (opts1: { x: string; y?: string | undefined; z?: string; w?: string; }) => void
 >{x: 'abc'} : { x: string; }
 >x : string
 >'abc' : "abc"
@@ -45,20 +45,20 @@ foo1({x: 'abc'});
  * @param {string=} opts2[].anotherY
  */
 function foo2(/** @param opts2 bad idea theatre! */opts2) {
->foo2 : (opts2: { anotherX: string; anotherY?: string; }[]) => void
->opts2 : { anotherX: string; anotherY?: string; }[]
+>foo2 : (opts2: { anotherX: string; anotherY?: string | undefined; }[]) => void
+>opts2 : { anotherX: string; anotherY?: string | undefined; }[]
 
     opts2[0].anotherX;
 >opts2[0].anotherX : string
->opts2[0] : { anotherX: string; anotherY?: string; }
->opts2 : { anotherX: string; anotherY?: string; }[]
+>opts2[0] : { anotherX: string; anotherY?: string | undefined; }
+>opts2 : { anotherX: string; anotherY?: string | undefined; }[]
 >0 : 0
 >anotherX : string
 }
 
 foo2([{anotherX: "world"}]);
 >foo2([{anotherX: "world"}]) : void
->foo2 : (opts2: { anotherX: string; anotherY?: string; }[]) => void
+>foo2 : (opts2: { anotherX: string; anotherY?: string | undefined; }[]) => void
 >[{anotherX: "world"}] : { anotherX: string; }[]
 >{anotherX: "world"} : { anotherX: string; }
 >anotherX : string
@@ -92,20 +92,20 @@ foo3({x: 'abc'});
  * @param {string} [opts4[].w="hi"]
  */
 function foo4(opts4) {
->foo4 : (opts4: { x: string; y?: string; z?: string; w?: string; }[]) => void
->opts4 : { x: string; y?: string; z?: string; w?: string; }[]
+>foo4 : (opts4: { x: string; y?: string | undefined; z?: string; w?: string; }[]) => void
+>opts4 : { x: string; y?: string | undefined; z?: string; w?: string; }[]
 
     opts4[0].x;
 >opts4[0].x : string
->opts4[0] : { x: string; y?: string; z?: string; w?: string; }
->opts4 : { x: string; y?: string; z?: string; w?: string; }[]
+>opts4[0] : { x: string; y?: string | undefined; z?: string; w?: string; }
+>opts4 : { x: string; y?: string | undefined; z?: string; w?: string; }[]
 >0 : 0
 >x : string
 }
 
 foo4([{ x: 'hi' }]);
 >foo4([{ x: 'hi' }]) : void
->foo4 : (opts4: { x: string; y?: string; z?: string; w?: string; }[]) => void
+>foo4 : (opts4: { x: string; y?: string | undefined; z?: string; w?: string; }[]) => void
 >[{ x: 'hi' }] : { x: string; }[]
 >{ x: 'hi' } : { x: string; }
 >x : string
diff --git a/tests/baselines/reference/paramTagBracketsAddOptionalUndefined.symbols b/tests/baselines/reference/paramTagBracketsAddOptionalUndefined.symbols
new file mode 100644
index 0000000000000..fd2e2ceeedad2
--- /dev/null
+++ b/tests/baselines/reference/paramTagBracketsAddOptionalUndefined.symbols
@@ -0,0 +1,38 @@
+=== tests/cases/conformance/jsdoc/a.js ===
+/**
+ * @param {number} [p]
+ * @param {number=} q
+ * @param {number} [r=101]
+ */
+function f(p, q, r) {
+>f : Symbol(f, Decl(a.js, 0, 0))
+>p : Symbol(p, Decl(a.js, 5, 11))
+>q : Symbol(q, Decl(a.js, 5, 13))
+>r : Symbol(r, Decl(a.js, 5, 16))
+
+    p = undefined
+>p : Symbol(p, Decl(a.js, 5, 11))
+>undefined : Symbol(undefined)
+
+    q = undefined
+>q : Symbol(q, Decl(a.js, 5, 13))
+>undefined : Symbol(undefined)
+
+    // note that, unlike TS, JSDOC [r=101] retains | undefined because
+    // there's no code emitted to get rid of it.
+    r = undefined
+>r : Symbol(r, Decl(a.js, 5, 16))
+>undefined : Symbol(undefined)
+}
+f()
+>f : Symbol(f, Decl(a.js, 0, 0))
+
+f(undefined, undefined, undefined)
+>f : Symbol(f, Decl(a.js, 0, 0))
+>undefined : Symbol(undefined)
+>undefined : Symbol(undefined)
+>undefined : Symbol(undefined)
+
+f(1, 2, 3)
+>f : Symbol(f, Decl(a.js, 0, 0))
+
diff --git a/tests/baselines/reference/paramTagBracketsAddOptionalUndefined.types b/tests/baselines/reference/paramTagBracketsAddOptionalUndefined.types
new file mode 100644
index 0000000000000..f8f531b3e2756
--- /dev/null
+++ b/tests/baselines/reference/paramTagBracketsAddOptionalUndefined.types
@@ -0,0 +1,47 @@
+=== tests/cases/conformance/jsdoc/a.js ===
+/**
+ * @param {number} [p]
+ * @param {number=} q
+ * @param {number} [r=101]
+ */
+function f(p, q, r) {
+>f : (p?: number | undefined, q?: number | undefined, r?: number | undefined) => void
+>p : number | undefined
+>q : number | undefined
+>r : number | undefined
+
+    p = undefined
+>p = undefined : undefined
+>p : number | undefined
+>undefined : undefined
+
+    q = undefined
+>q = undefined : undefined
+>q : number | undefined
+>undefined : undefined
+
+    // note that, unlike TS, JSDOC [r=101] retains | undefined because
+    // there's no code emitted to get rid of it.
+    r = undefined
+>r = undefined : undefined
+>r : number | undefined
+>undefined : undefined
+}
+f()
+>f() : void
+>f : (p?: number | undefined, q?: number | undefined, r?: number | undefined) => void
+
+f(undefined, undefined, undefined)
+>f(undefined, undefined, undefined) : void
+>f : (p?: number | undefined, q?: number | undefined, r?: number | undefined) => void
+>undefined : undefined
+>undefined : undefined
+>undefined : undefined
+
+f(1, 2, 3)
+>f(1, 2, 3) : void
+>f : (p?: number | undefined, q?: number | undefined, r?: number | undefined) => void
+>1 : 1
+>2 : 2
+>3 : 3
+
diff --git a/tests/cases/conformance/jsdoc/paramTagBracketsAddOptionalUndefined.ts b/tests/cases/conformance/jsdoc/paramTagBracketsAddOptionalUndefined.ts
new file mode 100644
index 0000000000000..e7ba7feae4629
--- /dev/null
+++ b/tests/cases/conformance/jsdoc/paramTagBracketsAddOptionalUndefined.ts
@@ -0,0 +1,21 @@
+// @noEmit: true
+// @allowJs: true
+// @checkJs: true
+// @strict: true
+// @Filename: a.js
+
+/**
+ * @param {number} [p]
+ * @param {number=} q
+ * @param {number} [r=101]
+ */
+function f(p, q, r) {
+    p = undefined
+    q = undefined
+    // note that, unlike TS, JSDOC [r=101] retains | undefined because
+    // there's no code emitted to get rid of it.
+    r = undefined
+}
+f()
+f(undefined, undefined, undefined)
+f(1, 2, 3)