diff --git a/changelog.d/15004.feature b/changelog.d/15004.feature
new file mode 100644
index 000000000000..d11d0aca91da
--- /dev/null
+++ b/changelog.d/15004.feature
@@ -0,0 +1 @@
+Implement [MSC3873](https://github.com/matrix-org/matrix-spec-proposals/pull/3873) to unambiguate push rule keys with dots in them.
diff --git a/rust/src/push/base_rules.rs b/rust/src/push/base_rules.rs
index e9af26dd4f8f..5c7ec11894e9 100644
--- a/rust/src/push/base_rules.rs
+++ b/rust/src/push/base_rules.rs
@@ -71,7 +71,7 @@ pub const BASE_APPEND_OVERRIDE_RULES: &[PushRule] = &[
         priority_class: 5,
         conditions: Cow::Borrowed(&[Condition::Known(KnownCondition::EventMatch(
             EventMatchCondition {
-                key: Cow::Borrowed("content.m.relates_to.rel_type"),
+                key: Cow::Borrowed("content.m\\.relates_to.rel_type"),
                 pattern: Some(Cow::Borrowed("m.replace")),
                 pattern_type: None,
             },
diff --git a/synapse/push/bulk_push_rule_evaluator.py b/synapse/push/bulk_push_rule_evaluator.py
index d9c0a98f44ee..f6122e8d651e 100644
--- a/synapse/push/bulk_push_rule_evaluator.py
+++ b/synapse/push/bulk_push_rule_evaluator.py
@@ -521,6 +521,11 @@ def _flatten_dict(
     if result is None:
         result = {}
     for key, value in d.items():
+        # Escape periods in the key with a backslash (and backslashes with an
+        # extra backslash). This is since a period is used as a separator between
+        # nested fields.
+        key = key.replace("\\", "\\\\").replace(".", "\\.")
+
         if isinstance(value, str):
             result[".".join(prefix + [key])] = value.lower()
         elif isinstance(value, Mapping):
diff --git a/tests/push/test_push_rule_evaluator.py b/tests/push/test_push_rule_evaluator.py
index da3342387121..ee36d6163ae6 100644
--- a/tests/push/test_push_rule_evaluator.py
+++ b/tests/push/test_push_rule_evaluator.py
@@ -48,6 +48,10 @@ def test_nested(self) -> None:
         input = {"foo": {"bar": "abc"}}
         self.assertEqual({"foo.bar": "abc"}, _flatten_dict(input))
 
+        # If a field has a dot in it, escape it.
+        input = {"m.foo": {"b\\ar": "abc"}}
+        self.assertEqual({"m\\.foo.b\\\\ar": "abc"}, _flatten_dict(input))
+
     def test_non_string(self) -> None:
         """Non-string items are dropped."""
         input: Dict[str, Any] = {