diff --git a/crates/oxc_codegen/src/gen.rs b/crates/oxc_codegen/src/gen.rs
index 11b87e2dd4f8d..ab137e29568c4 100644
--- a/crates/oxc_codegen/src/gen.rs
+++ b/crates/oxc_codegen/src/gen.rs
@@ -3032,17 +3032,28 @@ impl Gen for TSTupleType<'_> {
     }
 }
 
+fn parenthesize_check_type_of_conditional_type(ty: &TSType<'_>) -> bool {
+    matches!(
+        ty,
+        TSType::TSFunctionType(_) | TSType::TSConstructorType(_) | TSType::TSConditionalType(_)
+    )
+}
+
 impl Gen for TSUnionType<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         let Some((first, rest)) = self.types.split_first() else {
             return;
         };
-        first.print(p, ctx);
+        p.wrap(parenthesize_check_type_of_conditional_type(first), |p| {
+            first.print(p, ctx);
+        });
         for item in rest {
             p.print_soft_space();
             p.print_str("|");
             p.print_soft_space();
-            item.print(p, ctx);
+            p.wrap(parenthesize_check_type_of_conditional_type(item), |p| {
+                item.print(p, ctx);
+            });
         }
     }
 }
diff --git a/crates/oxc_isolated_declarations/tests/fixtures/union-type.ts b/crates/oxc_isolated_declarations/tests/fixtures/union-type.ts
new file mode 100644
index 0000000000000..3e304d83b0143
--- /dev/null
+++ b/crates/oxc_isolated_declarations/tests/fixtures/union-type.ts
@@ -0,0 +1,7 @@
+export class Foo {
+  constructor(
+    public one?: () => void,
+    protected two?: () => void,
+    private three?: () => void,
+  ) {}
+}
diff --git a/crates/oxc_isolated_declarations/tests/snapshots/union-type.snap b/crates/oxc_isolated_declarations/tests/snapshots/union-type.snap
new file mode 100644
index 0000000000000..9b95e446e04f6
--- /dev/null
+++ b/crates/oxc_isolated_declarations/tests/snapshots/union-type.snap
@@ -0,0 +1,13 @@
+---
+source: crates/oxc_isolated_declarations/tests/mod.rs
+input_file: crates/oxc_isolated_declarations/tests/fixtures/union-type.ts
+---
+```
+==================== .D.TS ====================
+
+export declare class Foo {
+	one?: (() => void) | undefined;
+	protected two?: (() => void) | undefined;
+	private three?;
+	constructor(one?: (() => void) | undefined, two?: (() => void) | undefined, three?: (() => void) | undefined);
+}
diff --git a/tasks/coverage/snapshots/transpile.snap b/tasks/coverage/snapshots/transpile.snap
index a25cc194d81c3..8960fa9eb99ca 100644
--- a/tasks/coverage/snapshots/transpile.snap
+++ b/tasks/coverage/snapshots/transpile.snap
@@ -395,8 +395,8 @@ export class ClsWithRequiredInitializedParameter {
 //// [fnDecl.d.ts] ////
 type T = number[];
 export declare function fnDeclBasic1(p: number[] | string[] | [T] | undefined, rParam: string): void;
-export declare function fnDeclBasic2(p: (n: T) => T | undefined, rParam: string): void;
-export declare function fnDeclBasic3(p: new () => any | undefined, rParam: string): void;
+export declare function fnDeclBasic2(p: ((n: T) => T) | undefined, rParam: string): void;
+export declare function fnDeclBasic3(p: (new () => any) | undefined, rParam: string): void;
 export declare function fnDeclBasic4(p: [T] | undefined, rParam: string): void;
 export declare function fnDeclBasic5(p: {
 	a: T;