diff --git a/temporalio/converter.py b/temporalio/converter.py
index cd7c2bbf5..190fda0e6 100644
--- a/temporalio/converter.py
+++ b/temporalio/converter.py
@@ -1590,7 +1590,9 @@ def value_to_type(
                         elif key_type is type(None):
                             key = {"null": None}[key]
 
-                    if not isinstance(key, key_type):
+                    # Can't call isinstance if key_type is a newtype
+                    is_newtype = getattr(key_type, "__supertype__", None)
+                    if is_newtype or not isinstance(key, key_type):
                         key = value_to_type(key_type, key, custom_converters)
                 except Exception as err:
                     raise TypeError(
diff --git a/tests/test_converter.py b/tests/test_converter.py
index 94fff5725..e274b10d9 100644
--- a/tests/test_converter.py
+++ b/tests/test_converter.py
@@ -53,6 +53,7 @@
     _JSONTypeConverterUnhandled,
     decode_search_attributes,
     encode_search_attribute_values,
+    value_to_type,
 )
 from temporalio.exceptions import ApplicationError, FailureError
 
@@ -91,6 +92,14 @@ class DatetimeClass:
     datetime: datetime
 
 
+MyNewTypeStr = NewType("MyNewTypeStr", str)
+
+
+@dataclass
+class NewTypeMessage:
+    data: dict[MyNewTypeStr, str]
+
+
 async def test_converter_default():
     async def assert_payload(
         input,
@@ -215,6 +224,22 @@ async def assert_payload(
         type_hint=DatetimeClass,
     )
 
+    # Newtype String
+    await assert_payload(
+        MyNewTypeStr("somestr"),
+        "json/plain",
+        '"somestr"',
+        type_hint=MyNewTypeStr,
+    )
+
+    # Newtype String key
+    await assert_payload(
+        NewTypeMessage({MyNewTypeStr("key"): "value"}),
+        "json/plain",
+        '{"data":{"key":"value"}}',
+        type_hint=NewTypeMessage,
+    )
+
 
 def test_binary_proto():
     # We have to test this separately because by default it never encodes