RustPython
diff --git a/Lib/test/test_builtin.py b/Lib/test/test_builtin.py
@@ -2012,21 +2012,18 @@ def test_construct_singletons(self):
             self.assertRaises(TypeError, tp, 1, 2)
             self.assertRaises(TypeError, tp, a=1, b=2)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
-    def test_warning_notimplemented(self):
-        # Issue #35712: NotImplemented is a sentinel value that should never
+    def test_bool_notimplemented(self):
+        # GH-79893: NotImplemented is a sentinel value that should never
         # be evaluated in a boolean context (virtually all such use cases
         # are a result of accidental misuse implementing rich comparison
         # operations in terms of one another).
-        # For the time being, it will continue to evaluate as a true value, but
-        # issue a deprecation warning (with the eventual intent to make it
-        # a TypeError).
-        self.assertWarns(DeprecationWarning, bool, NotImplemented)
-        with self.assertWarns(DeprecationWarning):
-            self.assertTrue(NotImplemented)
-        with self.assertWarns(DeprecationWarning):
-            self.assertFalse(not NotImplemented)
+        msg = "NotImplemented should not be used in a boolean context"
+        self.assertRaisesRegex(TypeError, msg, bool, NotImplemented)
+        with self.assertRaisesRegex(TypeError, msg):
+            if NotImplemented:
+                pass
+        with self.assertRaisesRegex(TypeError, msg):
+            not NotImplemented
 
 
 class TestBreakpoint(unittest.TestCase):

diff --git a/crates/vm/src/builtins/singletons.rs b/crates/vm/src/builtins/singletons.rs
@@ -103,14 +103,11 @@ impl Constructor for PyNotImplemented {
     }
 }
 
-#[pyclass(with(Constructor))]
+#[pyclass(with(Constructor, AsNumber))]
 impl PyNotImplemented {
-    // TODO: As per https://bugs.python.org/issue35712, using NotImplemented
-    // in boolean contexts will need to raise a DeprecationWarning in 3.9
-    // and, eventually, a TypeError.
     #[pymethod]
-    const fn __bool__(&self) -> bool {
-        true
+    fn __bool__(&self, vm: &VirtualMachine) -> PyResult<bool> {
+        Err(vm.new_type_error("NotImplemented should not be used in a boolean context".to_owned()))
     }
 
     #[pymethod]
@@ -119,6 +116,20 @@ impl PyNotImplemented {
     }
 }
 
+impl AsNumber for PyNotImplemented {
+    fn as_number() -> &'static PyNumberMethods {
+        static AS_NUMBER: PyNumberMethods = PyNumberMethods {
+            boolean: Some(|_number, vm| {
+                Err(vm.new_type_error(
+                    "NotImplemented should not be used in a boolean context".to_owned(),
+                ))
+            }),
+            ..PyNumberMethods::NOT_IMPLEMENTED
+        };
+        &AS_NUMBER
+    }
+}
+
 impl Representable for PyNotImplemented {
     #[inline]
     fn repr(_zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<PyStrRef> {

diff --git a/extra_tests/snippets/builtin_bool.py b/extra_tests/snippets/builtin_bool.py
@@ -18,7 +18,9 @@
 assert bool(1) is True
 assert bool({}) is False
 
-assert bool(NotImplemented) is True
+# NotImplemented cannot be used in a boolean context (Python 3.14+)
+with assert_raises(TypeError):
+    bool(NotImplemented)
 assert bool(...) is True
 
 if not 1: