diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index b599f25..ede16a7 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -33,6 +33,8 @@ vNext
 Fixes
 ~~~~~
 
+* **Postmark:** Fix API error when sending with template to single recipient.
+  (Thanks to `@jc-ee`_ for finding and fixing the issue.)
 * **SendGrid:** Allow non-batch template send to multiple recipients when
   `merge_global_data` is set without `merge_data`. (Broken in v6.0. Thanks to
   `@vgrebenschikov`_ for the bug report.)
@@ -1026,6 +1028,7 @@ Features
 .. _@ewingrj: https://github.com/ewingrj
 .. _@fdemmer: https://github.com/fdemmer
 .. _@janneThoft: https://github.com/janneThoft
+.. _@jc-ee: https://github.com/jc-ee
 .. _@joshkersey: https://github.com/joshkersey
 .. _@Lekensteyn: https://github.com/Lekensteyn
 .. _@lewistaylor: https://github.com/lewistaylor
diff --git a/anymail/backends/postmark.py b/anymail/backends/postmark.py
index 58a9a65..2cae34c 100644
--- a/anymail/backends/postmark.py
+++ b/anymail/backends/postmark.py
@@ -179,12 +179,19 @@ class PostmarkPayload(RequestsPayload):
         return params
 
     def serialize_data(self):
-        data = self.data
         api_endpoint = self.get_api_endpoint()
-        if api_endpoint == "email/batchWithTemplates":
+        if api_endpoint == "email":
+            data = self.data
+        elif api_endpoint == "email/batchWithTemplates":
             data = {"Messages": [self.data_for_recipient(to) for to in self.to_emails]}
         elif api_endpoint == "email/batch":
             data = [self.data_for_recipient(to) for to in self.to_emails]
+        elif api_endpoint == "email/withTemplate/":
+            assert len(self.to_emails) == 1
+            data = self.data_for_recipient(self.to_emails[0])
+        else:
+            raise AssertionError("PostmarkPayload.serialize_data missing"
+                                 " case for api_endpoint %r" % api_endpoint)
         return self.serialize_json(data)
 
     def data_for_recipient(self, to):
diff --git a/tests/test_postmark_backend.py b/tests/test_postmark_backend.py
index 3629171..3c28a18 100644
--- a/tests/test_postmark_backend.py
+++ b/tests/test_postmark_backend.py
@@ -450,6 +450,34 @@ class PostmarkBackendAnymailFeatureTests(PostmarkBackendMockAPITestCase):
         self.assertEqual(recipients['bob@example.com'].status, 'sent')
         self.assertEqual(recipients['bob@example.com'].message_id, 'e2ecbbfc-fe12-463d-b933-9fe22915106d')
 
+    def test_merge_data_single_recipient(self):
+        self.set_mock_response(raw=self._mock_batch_response)
+        message = AnymailMessage(
+            from_email='from@example.com',
+            template_id=1234567,  # Postmark only supports merge_data content in a template
+            to=['alice@example.com'],
+            merge_data={
+                'alice@example.com': {'name': "Alice", 'group': "Developers"},
+                'nobody@example.com': {'name': "Not a recipient for this message"},
+            },
+            merge_global_data={'group': "Users", 'site': "ExampleCo"}
+        )
+        message.send()
+
+        self.assert_esp_called('/email/withTemplate/')
+        data = self.get_api_call_json()
+
+        self.assertEqual(data, {
+            "From": "from@example.com",
+            "To": "alice@example.com",
+            "TemplateId": 1234567,
+            "TemplateModel": {"name": "Alice", "group": "Developers", "site": "ExampleCo"},
+        })
+
+        recipients = message.anymail_status.recipients
+        self.assertEqual(recipients['alice@example.com'].status, 'sent')
+        self.assertEqual(recipients['alice@example.com'].message_id, 'b7bc2f4a-e38e-4336-af7d-e6c392c2f817')
+
     def test_merge_data_no_template(self):
         # merge_data={} can be used to force batch sending without a template
         self.set_mock_response(raw=self._mock_batch_response)