diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index c93f984..1d35935 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -39,6 +39,9 @@ Features
 *  **SendGrid:** Support both new "dynamic" and original "legacy" transactional
    templates. (See
    `docs <https://anymail.readthedocs.io/en/latest/esps/sendgrid/#sendgrid-templates>`__.)
+*  **SendGrid:** Allow merging `esp_extra["personalizations"]` dict into other message-derived
+   personalizations. (See
+   `docs <https://anymail.readthedocs.io/en/latest/esps/sendgrid/#sendgrid-esp-extra>`__.)
 
 
 v4.0
diff --git a/anymail/backends/sendgrid.py b/anymail/backends/sendgrid.py
index 4210a0c..5238df4 100644
--- a/anymail/backends/sendgrid.py
+++ b/anymail/backends/sendgrid.py
@@ -1,4 +1,5 @@
 import uuid
+from collections import Mapping
 from email.utils import quote as rfc822_quote
 import warnings
 
@@ -171,7 +172,7 @@ class SendGridPayload(RequestsPayload):
                     pass  # no merge_data for this recipient
                 self.data["personalizations"].append(personalization)
 
-            if self.merge_field_format is None and all(field.isalnum() for field in all_fields):
+            if self.merge_field_format is None and len(all_fields) and all(field.isalnum() for field in all_fields):
                 warnings.warn(
                     "Your SendGrid merge fields don't seem to have delimiters, "
                     "which can cause unexpected results with Anymail's merge_data. "
@@ -347,6 +348,10 @@ class SendGridPayload(RequestsPayload):
     def set_esp_extra(self, extra):
         self.merge_field_format = extra.pop("merge_field_format", self.merge_field_format)
         self.use_dynamic_template = extra.pop("use_dynamic_template", self.use_dynamic_template)
+        if isinstance(extra.get("personalizations", None), Mapping):
+            # merge personalizations *dict* into other message personalizations
+            assert len(self.data["personalizations"]) == 1
+            self.data["personalizations"][0].update(extra.pop("personalizations"))
         if "x-smtpapi" in extra:
             raise AnymailConfigurationError(
                 "You are attempting to use SendGrid v2 API-style x-smtpapi params "
diff --git a/docs/esps/sendgrid.rst b/docs/esps/sendgrid.rst
index 41d0b43..34bf49f 100644
--- a/docs/esps/sendgrid.rst
+++ b/docs/esps/sendgrid.rst
@@ -123,6 +123,12 @@ Your :attr:`esp_extra` dict will be deeply merged into the
 parameters Anymail has constructed for the send, with `esp_extra`
 having precedence in conflicts.
 
+Anymail has special handling for `esp_extra["personalizations"]`. If that value
+is a `dict`, Anymail will merge that personalizations dict into the personalizations
+for each message recipient. (If you pass a `list`, that will override the
+personalizations Anymail normally constructs from the message, and you will need to
+specify each recipient in the personalizations list yourself.)
+
 Example:
 
     .. code-block:: python
@@ -140,6 +146,11 @@ Example:
                     "substitution_tag": "%%OPEN_TRACKING_PIXEL%%",
                 },
             },
+            # Because "personalizations" is a dict, Anymail will merge "future_feature"
+            # into the SendGrid personalizations array for each message recipient
+            "personalizations": {
+                "future_feature": {"future": "data"},
+            },
         }
 
 
diff --git a/tests/test_sendgrid_backend.py b/tests/test_sendgrid_backend.py
index b72352f..9680ba9 100644
--- a/tests/test_sendgrid_backend.py
+++ b/tests/test_sendgrid_backend.py
@@ -627,6 +627,34 @@ class SendGridBackendAnymailFeatureTests(SendGridBackendMockAPITestCase):
         self.assertEqual(data['categories'], ["tag"])
         self.assertEqual(data['tracking_settings']['click_tracking'], {'enable': True})
 
+    def test_esp_extra_pesonalizations(self):
+        self.message.to = ["First recipient <first@example.com>", "second@example.com"]
+        self.message.merge_data = {}  # force separate messages for each 'to'
+
+        # esp_extra['personalizations'] dict merges with message-derived personalizations
+        self.message.esp_extra = {
+            "personalizations": {"future_feature": "works"}}
+        self.message.send()
+        data = self.get_api_call_json()
+        self.assertEqual(data['personalizations'], [
+            {'to': [{'email': 'first@example.com', 'name': '"First recipient"'}],
+             'future_feature': "works"},
+            {'to': [{'email': 'second@example.com'}],
+             'future_feature': "works"},  # merged into *every* recipient
+        ])
+
+        # but esp_extra['personalizations'] list just overrides entire personalizations
+        # (for backwards compatibility)
+        self.message.esp_extra = {
+            "personalizations": [{"to": [{"email": "custom@example.com"}],
+                                  "future_feature": "works"}]}
+        self.message.send()
+        data = self.get_api_call_json()
+        self.assertEqual(data['personalizations'], [
+            {'to': [{'email': 'custom@example.com'}],
+             'future_feature': "works"},
+        ])
+
     # noinspection PyUnresolvedReferences
     def test_send_attaches_anymail_status(self):
         """ The anymail_status should be attached to the message when it is sent """