diff --git a/changelog.d/20443.feature b/changelog.d/20443.feature
new file mode 100644
index 0000000000..fe776af627
--- /dev/null
+++ b/changelog.d/20443.feature
@@ -0,0 +1 @@
+Implement experimental support for [MSC4506](https://github.com/matrix-org/matrix-spec-proposals/pull/4506): notify relevant users if there's a knock on a room.
diff --git a/rust/benches/evaluator.rs b/rust/benches/evaluator.rs
index 4405932750..b9e66779a7 100644
--- a/rust/benches/evaluator.rs
+++ b/rust/benches/evaluator.rs
@@ -235,6 +235,7 @@ fn bench_eval_message(b: &mut Bencher) {
         false,
         false,
         false,
+        false,
     );
 
     b.iter(|| eval.run(&rules, Some("bob"), Some("person"), None, None));
diff --git a/rust/src/push/base_rules.rs b/rust/src/push/base_rules.rs
index fbc985a2f0..a458bcf053 100644
--- a/rust/src/push/base_rules.rs
+++ b/rust/src/push/base_rules.rs
@@ -117,6 +117,30 @@ pub const BASE_APPEND_OVERRIDE_RULES: &[PushRule] = &[
         default: true,
         default_enabled: true,
     },
+    // MSC4506: notify the members of a room who are able to act on a knock
+    // (i.e. those with a power level sufficient to invite the knocker). Must
+    // come before `.m.rule.member_event`, which suppresses all other member
+    // events.
+    PushRule {
+        rule_id: Cow::Borrowed("global/override/.org.matrix.msc4506.rule.knock"),
+        priority_class: 5,
+        conditions: Cow::Borrowed(&[
+            Condition::Known(KnownCondition::EventMatch(EventMatchCondition {
+                key: Cow::Borrowed("type"),
+                pattern: Cow::Borrowed("m.room.member"),
+            })),
+            Condition::Known(KnownCondition::EventMatch(EventMatchCondition {
+                key: Cow::Borrowed("content.membership"),
+                pattern: Cow::Borrowed("knock"),
+            })),
+            Condition::Known(KnownCondition::RecipientPermission {
+                key: Cow::Borrowed("invite"),
+            }),
+        ]),
+        actions: Cow::Borrowed(&[Action::Notify, HIGHLIGHT_FALSE_ACTION, SOUND_ACTION]),
+        default: true,
+        default_enabled: true,
+    },
     PushRule {
         rule_id: Cow::Borrowed("global/override/.m.rule.member_event"),
         priority_class: 5,
@@ -811,6 +835,7 @@ mod tests {
             msc4075_enabled,
             false, // msc4210_enabled
             false, // msc4306_enabled
+            false, // msc4506_enabled
         )
     }
 
diff --git a/rust/src/push/evaluator.rs b/rust/src/push/evaluator.rs
index 63e149315f..9babc0b0d0 100644
--- a/rust/src/push/evaluator.rs
+++ b/rust/src/push/evaluator.rs
@@ -730,6 +730,7 @@ fn test_requires_room_version_supports_condition() {
             false,
             false,
             false,
+            false,
         ),
         None,
         None,
diff --git a/rust/src/push/mod.rs b/rust/src/push/mod.rs
index 480b1ccf40..c972da91c6 100644
--- a/rust/src/push/mod.rs
+++ b/rust/src/push/mod.rs
@@ -561,6 +561,7 @@ pub struct FilteredPushRules {
     msc4075_enabled: bool,
     msc4210_enabled: bool,
     msc4306_enabled: bool,
+    msc4506_enabled: bool,
 }
 
 #[pymethods]
@@ -577,6 +578,7 @@ impl FilteredPushRules {
         msc4075_enabled: bool,
         msc4210_enabled: bool,
         msc4306_enabled: bool,
+        msc4506_enabled: bool,
     ) -> Self {
         Self {
             push_rules,
@@ -588,6 +590,7 @@ impl FilteredPushRules {
             msc4075_enabled,
             msc4210_enabled,
             msc4306_enabled,
+            msc4506_enabled,
         }
     }
 
@@ -624,6 +627,11 @@ impl FilteredPushRules {
                     return false;
                 }
 
+                if !self.msc4506_enabled && rule.rule_id.contains("/.org.matrix.msc4506.rule.knock")
+                {
+                    return false;
+                }
+
                 if !self.msc4028_push_encrypted_events
                     && rule.rule_id == "global/override/.org.matrix.msc4028.encrypted_event"
                 {
diff --git a/synapse/push/bulk_push_rule_evaluator.py b/synapse/push/bulk_push_rule_evaluator.py
index a880ab0faa..7b5341fe0c 100644
--- a/synapse/push/bulk_push_rule_evaluator.py
+++ b/synapse/push/bulk_push_rule_evaluator.py
@@ -192,6 +192,15 @@ class BulkPushRuleEvaluator:
                 )
                 if target_already_in_room:
                     local_users = [event.state_key]
+            elif (
+                self.hs.config.experimental.msc4506_enabled
+                and event.membership == Membership.KNOCK
+            ):
+                # MSC4506: a knock (sender == state_key, so not caught above)
+                # should notify the members of the room who can act on it, so
+                # we can't take the membership-events-only-notify-their-target
+                # fast path.
+                local_users = await self.store.get_local_users_in_room(event.room_id)
         else:
             # We get the users who may need to be notified by first fetching the
             # local users currently in the room, finding those that have push rules,
diff --git a/synapse/storage/databases/main/push_rule.py b/synapse/storage/databases/main/push_rule.py
index b65f5e8bfa..7a11483cf4 100644
--- a/synapse/storage/databases/main/push_rule.py
+++ b/synapse/storage/databases/main/push_rule.py
@@ -128,6 +128,7 @@ def filter_push_rules(
         msc4075_enabled=experimental_config.msc4075_enabled,
         msc4210_enabled=experimental_config.msc4210_enabled,
         msc4306_enabled=experimental_config.msc4306_enabled,
+        msc4506_enabled=experimental_config.msc4506_enabled,
     )
 
 
diff --git a/synapse/synapse_rust/push.pyi b/synapse/synapse_rust/push.pyi
index e677cf3841..80f029195d 100644
--- a/synapse/synapse_rust/push.pyi
+++ b/synapse/synapse_rust/push.pyi
@@ -51,6 +51,7 @@ class FilteredPushRules:
         msc4075_enabled: bool,
         msc4210_enabled: bool,
         msc4306_enabled: bool,
+        msc4506_enabled: bool,
     ): ...
     def rules(self) -> Collection[tuple[PushRule, bool]]: ...