diff --git a/packages/pyright-internal/src/analyzer/checker.ts b/packages/pyright-internal/src/analyzer/checker.ts
index 460d13400..28d9c4332 100644
--- a/packages/pyright-internal/src/analyzer/checker.ts
+++ b/packages/pyright-internal/src/analyzer/checker.ts
@@ -6337,6 +6337,12 @@ export class Checker extends ParseTreeWalker {
                                 type: this._evaluator.printType(entry.valueType),
                             })
                         );
+                    } else if (!baseTypedDictEntries.extraItems.isReadOnly && entry.isReadOnly) {
+                        diag.addMessage(
+                            LocAddendum.typedDictClosedFieldNotReadOnly().format({
+                                name,
+                            })
+                        );
                     } else if (!baseTypedDictEntries.extraItems.isReadOnly && entry.isRequired) {
                         diag.addMessage(
                             LocAddendum.typedDictClosedFieldNotRequired().format({
diff --git a/packages/pyright-internal/src/localization/localize.ts b/packages/pyright-internal/src/localization/localize.ts
index a2ca2056e..fa30d3966 100644
--- a/packages/pyright-internal/src/localization/localize.ts
+++ b/packages/pyright-internal/src/localization/localize.ts
@@ -1563,6 +1563,10 @@ export namespace Localizer {
             new ParameterizedString<{ name: string; type: string }>(
                 getRawString('DiagnosticAddendum.typedDictClosedExtraTypeMismatch')
             );
+        export const typedDictClosedFieldNotReadOnly = () =>
+            new ParameterizedString<{ name: string }>(
+                getRawString('DiagnosticAddendum.typedDictClosedFieldNotReadOnly')
+            );
         export const typedDictClosedFieldNotRequired = () =>
             new ParameterizedString<{ name: string }>(
                 getRawString('DiagnosticAddendum.typedDictClosedFieldNotRequired')
diff --git a/packages/pyright-internal/src/localization/package.nls.en-us.json b/packages/pyright-internal/src/localization/package.nls.en-us.json
index df9cbe69c..63f1bebf8 100644
--- a/packages/pyright-internal/src/localization/package.nls.en-us.json
+++ b/packages/pyright-internal/src/localization/package.nls.en-us.json
@@ -2114,6 +2114,10 @@
         },
         "typedDictClosedExtraNotAllowed": "Cannot add item \"{name}\"",
         "typedDictClosedExtraTypeMismatch": "Cannot add item \"{name}\" with type \"{type}\"",
+        "typedDictClosedFieldNotReadOnly": {
+            "message": "Cannot add item \"{name}\" because it must be ReadOnly",
+            "comment": "{Locked='ReadOnly'}"
+        },
         "typedDictClosedFieldNotRequired": {
             "message": "Cannot add item \"{name}\" because it must be NotRequired",
             "comment": "{Locked='NotRequired'}"
diff --git a/packages/pyright-internal/src/tests/samples/typedDictClosed3.py b/packages/pyright-internal/src/tests/samples/typedDictClosed3.py
index 84a3b786e..235eec71a 100644
--- a/packages/pyright-internal/src/tests/samples/typedDictClosed3.py
+++ b/packages/pyright-internal/src/tests/samples/typedDictClosed3.py
@@ -125,15 +125,28 @@ class MovieWithYear(MovieBase):
 class ParentNonOpen5(TypedDict, closed=True):
     pass
 
+
 # This should generate an error because a subclass of
 # a closed TypedDict cannot be open.
 class ChildNotClosed5(ParentNonOpen5, closed=False):
     pass
 
+
 class ParentNonOpen6(TypedDict, extra_items=str):
     pass
 
+
 # This should generate an error because a subclass of
 # a closed TypedDict cannot be open.
 class ChildNotClosed6(ParentNonOpen6, closed=False):
     pass
+
+
+class ParentNonOpen7(TypedDict, extra_items=str):
+    pass
+
+
+# This should generate an error because added fields
+# cannot be ReadOnly.
+class ChildNotClosed7(ParentNonOpen7):
+    a: NotRequired[ReadOnly[str]]
diff --git a/packages/pyright-internal/src/tests/typeEvaluator5.test.ts b/packages/pyright-internal/src/tests/typeEvaluator5.test.ts
index d4ed2bd10..bf35f9c0e 100644
--- a/packages/pyright-internal/src/tests/typeEvaluator5.test.ts
+++ b/packages/pyright-internal/src/tests/typeEvaluator5.test.ts
@@ -360,7 +360,7 @@ test('TypedDictClosed2', () => {
 
 test('TypedDictClosed3', () => {
     const analysisResults = TestUtils.typeAnalyzeSampleFiles(['typedDictClosed3.py']);
-    TestUtils.validateResults(analysisResults, 12);
+    TestUtils.validateResults(analysisResults, 13);
 });
 
 test('TypedDictClosed4', () => {