diff --git a/crates/oxc_parser/src/diagnostics.rs b/crates/oxc_parser/src/diagnostics.rs
index 43050332baeb4..d7af4084946fc 100644
--- a/crates/oxc_parser/src/diagnostics.rs
+++ b/crates/oxc_parser/src/diagnostics.rs
@@ -701,6 +701,15 @@ pub fn cannot_appear_on_a_type_parameter(modifier: &Modifier) -> OxcDiagnostic {
         .with_label(modifier.span)
 }
 
+#[cold]
+pub fn can_only_appear_on_a_type_parameter_of_a_class_interface_or_type_alias(
+    modifier: ModifierKind,
+    span: Span,
+) -> OxcDiagnostic {
+    ts_error("1274", format!("'{modifier}' modifier can only appear on a type parameter of a class, interface or type alias."))
+        .with_label(span)
+}
+
 pub fn cannot_appear_on_a_parameter(modifier: &Modifier) -> OxcDiagnostic {
     ts_error("1090", format!("'{}' modifier cannot appear on a parameter.", modifier.kind))
         .with_label(modifier.span)
diff --git a/crates/oxc_parser/src/js/class.rs b/crates/oxc_parser/src/js/class.rs
index 001921a67a5da..25ac2f4a2f192 100644
--- a/crates/oxc_parser/src/js/class.rs
+++ b/crates/oxc_parser/src/js/class.rs
@@ -317,8 +317,16 @@ impl<'a> ParserImpl<'a> {
     }
 
     fn parse_class_element_name(&mut self, modifiers: &Modifiers<'a>) -> (PropertyKey<'a>, bool) {
-        if let Some(modifier) = modifiers.iter().find(|m| m.kind == ModifierKind::Const) {
-            self.error(diagnostics::const_class_member(modifier.span));
+        for modifier in modifiers.iter() {
+            match modifier.kind {
+                ModifierKind::Const => {
+                    self.error(diagnostics::const_class_member(modifier.span));
+                }
+                ModifierKind::In | ModifierKind::Out => {
+                    self.error(diagnostics::can_only_appear_on_a_type_parameter_of_a_class_interface_or_type_alias(modifier.kind, modifier.span));
+                }
+                _ => {}
+            }
         }
         match self.cur_kind() {
             Kind::PrivateIdentifier => {
diff --git a/crates/oxc_semantic/src/checker/typescript.rs b/crates/oxc_semantic/src/checker/typescript.rs
index 09c6d2fd03b97..4700888ed8a26 100644
--- a/crates/oxc_semantic/src/checker/typescript.rs
+++ b/crates/oxc_semantic/src/checker/typescript.rs
@@ -14,8 +14,39 @@ fn ts_error<M: Into<Cow<'static, str>>>(code: &'static str, message: M) -> OxcDi
     OxcDiagnostic::error(message).with_error_code("TS", code)
 }
 
+fn can_only_appear_on_a_type_parameter_of_a_class_interface_or_type_alias(
+    modifier: &str,
+    span: Span,
+) -> OxcDiagnostic {
+    ts_error("1274", format!("'{modifier}' modifier can only appear on a type parameter of a class, interface or type alias."))
+        .with_label(span)
+}
+
 pub fn check_ts_type_parameter<'a>(param: &TSTypeParameter<'a>, ctx: &SemanticBuilder<'a>) {
     check_type_name_is_reserved(&param.name, ctx, "Type parameter");
+    if param.r#in || param.out {
+        let is_allowed_node = matches!(
+            // skip parent TSTypeParameterDeclaration
+            ctx.nodes.ancestor_kinds(ctx.current_node_id).nth(1),
+            Some(
+                AstKind::TSInterfaceDeclaration(_)
+                    | AstKind::Class(_)
+                    | AstKind::TSTypeAliasDeclaration(_)
+            )
+        );
+        if !is_allowed_node {
+            if param.r#in {
+                ctx.error(can_only_appear_on_a_type_parameter_of_a_class_interface_or_type_alias(
+                    "in", param.span,
+                ));
+            }
+            if param.out {
+                ctx.error(can_only_appear_on_a_type_parameter_of_a_class_interface_or_type_alias(
+                    "out", param.span,
+                ));
+            }
+        }
+    }
 }
 
 /// '?' at the end of a type is not valid TypeScript syntax. Did you mean to write 'number | null | undefined'?(17019)
diff --git a/tasks/coverage/snapshots/parser_babel.snap b/tasks/coverage/snapshots/parser_babel.snap
index e21e83160f673..7edb7ad5ce584 100644
--- a/tasks/coverage/snapshots/parser_babel.snap
+++ b/tasks/coverage/snapshots/parser_babel.snap
@@ -14878,6 +14878,38 @@ Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/typesc
     ╰────
   help: Remove the duplicate modifier.
 
+  × TS(1274): 'in' modifier can only appear on a type parameter of a class, interface or type alias.
+     ╭─[babel/packages/babel-parser/test/fixtures/typescript/types/variance-annotations/input.ts:104:5]
+ 103 │ class C {
+ 104 │     in a = 0;  // Error
+     ·     ──
+ 105 │     out b = 0;  // Error
+     ╰────
+
+  × TS(1274): 'out' modifier can only appear on a type parameter of a class, interface or type alias.
+     ╭─[babel/packages/babel-parser/test/fixtures/typescript/types/variance-annotations/input.ts:105:5]
+ 104 │     in a = 0;  // Error
+ 105 │     out b = 0;  // Error
+     ·     ───
+ 106 │ }
+     ╰────
+
+  × TS(1274): 'in' modifier can only appear on a type parameter of a class, interface or type alias.
+     ╭─[babel/packages/babel-parser/test/fixtures/typescript/types/variance-annotations/input.ts:100:21]
+  99 │ 
+ 100 │ declare function f1<in T>(x: T): void;  // Error
+     ·                     ────
+ 101 │ declare function f2<out T>(): T;  // Error
+     ╰────
+
+  × TS(1274): 'out' modifier can only appear on a type parameter of a class, interface or type alias.
+     ╭─[babel/packages/babel-parser/test/fixtures/typescript/types/variance-annotations/input.ts:101:21]
+ 100 │ declare function f1<in T>(x: T): void;  // Error
+ 101 │ declare function f2<out T>(): T;  // Error
+     ·                     ─────
+ 102 │ 
+     ╰────
+
   × TS(1273): 'public' modifier cannot be used on a type parameter.
     ╭─[babel/packages/babel-parser/test/fixtures/typescript/types/variance-annotations-babel-7/input.ts:95:10]
  94 │ 
@@ -14904,6 +14936,38 @@ Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/typesc
     ╰────
   help: Remove the duplicate modifier.
 
+  × TS(1274): 'in' modifier can only appear on a type parameter of a class, interface or type alias.
+     ╭─[babel/packages/babel-parser/test/fixtures/typescript/types/variance-annotations-babel-7/input.ts:104:5]
+ 103 │ class C {
+ 104 │     in a = 0;  // Error
+     ·     ──
+ 105 │     out b = 0;  // Error
+     ╰────
+
+  × TS(1274): 'out' modifier can only appear on a type parameter of a class, interface or type alias.
+     ╭─[babel/packages/babel-parser/test/fixtures/typescript/types/variance-annotations-babel-7/input.ts:105:5]
+ 104 │     in a = 0;  // Error
+ 105 │     out b = 0;  // Error
+     ·     ───
+ 106 │ }
+     ╰────
+
+  × TS(1274): 'in' modifier can only appear on a type parameter of a class, interface or type alias.
+     ╭─[babel/packages/babel-parser/test/fixtures/typescript/types/variance-annotations-babel-7/input.ts:100:21]
+  99 │ 
+ 100 │ declare function f1<in T>(x: T): void;  // Error
+     ·                     ────
+ 101 │ declare function f2<out T>(): T;  // Error
+     ╰────
+
+  × TS(1274): 'out' modifier can only appear on a type parameter of a class, interface or type alias.
+     ╭─[babel/packages/babel-parser/test/fixtures/typescript/types/variance-annotations-babel-7/input.ts:101:21]
+ 100 │ declare function f1<in T>(x: T): void;  // Error
+ 101 │ declare function f2<out T>(): T;  // Error
+     ·                     ─────
+ 102 │ 
+     ╰────
+
   × Unexpected token. Did you mean `{'>'}` or `&gt;`?
    ╭─[babel/packages/babel-parser/test/fixtures/typescript/types/variance-annotations-with-jsx/input.tsx:2:11]
  1 │ // valid JSX
diff --git a/tasks/coverage/snapshots/parser_typescript.snap b/tasks/coverage/snapshots/parser_typescript.snap
index 5e2f7b2920566..92e37c4006029 100644
--- a/tasks/coverage/snapshots/parser_typescript.snap
+++ b/tasks/coverage/snapshots/parser_typescript.snap
@@ -27683,6 +27683,38 @@ Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/parser/ecmasc
     ╰────
   help: Remove the duplicate modifier.
 
+  × TS(1274): 'in' modifier can only appear on a type parameter of a class, interface or type alias.
+     ╭─[typescript/tests/cases/conformance/types/typeParameters/typeParameterLists/varianceAnnotations.ts:104:5]
+ 103 │ class C {
+ 104 │     in a = 0;  // Error
+     ·     ──
+ 105 │     out b = 0;  // Error
+     ╰────
+
+  × TS(1274): 'out' modifier can only appear on a type parameter of a class, interface or type alias.
+     ╭─[typescript/tests/cases/conformance/types/typeParameters/typeParameterLists/varianceAnnotations.ts:105:5]
+ 104 │     in a = 0;  // Error
+ 105 │     out b = 0;  // Error
+     ·     ───
+ 106 │ }
+     ╰────
+
+  × TS(1274): 'in' modifier can only appear on a type parameter of a class, interface or type alias.
+     ╭─[typescript/tests/cases/conformance/types/typeParameters/typeParameterLists/varianceAnnotations.ts:100:21]
+  99 │ 
+ 100 │ declare function f1<in T>(x: T): void;  // Error
+     ·                     ────
+ 101 │ declare function f2<out T>(): T;  // Error
+     ╰────
+
+  × TS(1274): 'out' modifier can only appear on a type parameter of a class, interface or type alias.
+     ╭─[typescript/tests/cases/conformance/types/typeParameters/typeParameterLists/varianceAnnotations.ts:101:21]
+ 100 │ declare function f1<in T>(x: T): void;  // Error
+ 101 │ declare function f2<out T>(): T;  // Error
+     ·                     ─────
+ 102 │ 
+     ╰────
+
   × Identifier expected. 'in' is a reserved word that cannot be used here.
    ╭─[typescript/tests/cases/conformance/types/typeParameters/typeParameterLists/varianceAnnotationsWithCircularlyReferencesError.ts:1:12]
  1 │ type T1<in in> = T1 // Error: circularly references