diff --git a/changelog.d/20104.bugfix b/changelog.d/20104.bugfix
new file mode 100644
index 0000000000..f04945660c
--- /dev/null
+++ b/changelog.d/20104.bugfix
@@ -0,0 +1 @@
+Fix a regression where `client_secret` request parameters were not validated against the character set required by the spec on Pydantic-validated endpoints, regressed in Synapse 1.66.0 (originally fixed for https://github.com/matrix-org/synapse/issues/6766).
diff --git a/synapse/types/rest/client/__init__.py b/synapse/types/rest/client/__init__.py
index 17647e621d..dbd4455b82 100644
--- a/synapse/types/rest/client/__init__.py
+++ b/synapse/types/rest/client/__init__.py
@@ -62,7 +62,7 @@ class AuthenticationData(RequestBodyModel):
 ClientSecretStr = Annotated[
     str,
     StringConstraints(
-        pattern="[0-9a-zA-Z.=_-]",
+        pattern="^[0-9a-zA-Z.=_-]+$",
         min_length=1,
         max_length=255,
         strict=True,
diff --git a/tests/rest/client/test_models.py b/tests/rest/client/test_models.py
index f297856830..e5dc922023 100644
--- a/tests/rest/client/test_models.py
+++ b/tests/rest/client/test_models.py
@@ -23,7 +23,7 @@ from typing import Literal
 
 from pydantic import BaseModel, ValidationError
 
-from synapse.types.rest.client import EmailRequestTokenBody
+from synapse.types.rest.client import ClientSecretStr, EmailRequestTokenBody
 
 
 class ThreepidMediumEnumTestCase(stdlib_unittest.TestCase):
@@ -48,6 +48,44 @@ class ThreepidMediumEnumTestCase(stdlib_unittest.TestCase):
             self.Model.model_validate({"medium": 123})
 
 
+class ClientSecretStrTestCase(stdlib_unittest.TestCase):
+    class Model(BaseModel):
+        client_secret: ClientSecretStr
+
+    def test_accepts_valid_client_secrets(self) -> None:
+        """Secrets consisting entirely of `[0-9a-zA-Z.=_-]` are accepted."""
+        for client_secret in (
+            "this.is-a_valid=secret",
+            "foobar",
+            "a",
+            "0123456789",
+            "a" * 255,
+        ):
+            with self.subTest(client_secret=client_secret):
+                model = self.Model.model_validate({"client_secret": client_secret})
+                self.assertEqual(model.client_secret, client_secret)
+
+    def test_rejects_client_secrets_with_invalid_characters(self) -> None:
+        for client_secret in (
+            "foo bar",
+            "secret!",
+            "café",
+            # Little bobby tables
+            "Robert'; DROP TABLE students;--",
+        ):
+            with self.subTest(client_secret=client_secret):
+                with self.assertRaises(ValidationError):
+                    self.Model.model_validate({"client_secret": client_secret})
+
+    def test_rejects_empty_client_secret(self) -> None:
+        with self.assertRaises(ValidationError):
+            self.Model.model_validate({"client_secret": ""})
+
+    def test_rejects_overlong_client_secret(self) -> None:
+        with self.assertRaises(ValidationError):
+            self.Model.model_validate({"client_secret": "a" * 256})
+
+
 class EmailRequestTokenBodyTestCase(stdlib_unittest.TestCase):
     base_request = {
         "client_secret": "hunter2",