diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0c27d222bf..0afd8d7026 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,12 @@
 
 * [CHANGE] TSDB: Fix the predicate checking for blocks which are beyond the retention period to include the ones right at the retention boundary. #9633
 
+## 2.51.2 / 2024-04-09
+
+Bugfix release.
+
+[BUGFIX] Notifier: could hang when using relabeling on alerts #13861
+
 ## 2.51.1 / 2024-03-27
 
 Bugfix release.
diff --git a/VERSION b/VERSION
index 8e00e241f3..587b583f91 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.51.1
+2.51.2
diff --git a/notifier/notifier.go b/notifier/notifier.go
index 53d3c04198..4cf376aa05 100644
--- a/notifier/notifier.go
+++ b/notifier/notifier.go
@@ -471,6 +471,10 @@ func (n *Manager) sendAll(alerts ...*Alert) bool {
 		numSuccess atomic.Uint64
 	)
 	for _, ams := range amSets {
+		if len(ams.ams) == 0 {
+			continue
+		}
+
 		var (
 			payload  []byte
 			err      error
@@ -482,6 +486,7 @@ func (n *Manager) sendAll(alerts ...*Alert) bool {
 		if len(ams.cfg.AlertRelabelConfigs) > 0 {
 			amAlerts = relabelAlerts(ams.cfg.AlertRelabelConfigs, labels.Labels{}, alerts)
 			if len(amAlerts) == 0 {
+				ams.mtx.RUnlock()
 				continue
 			}
 			// We can't use the cached values from previous iteration.
diff --git a/notifier/notifier_test.go b/notifier/notifier_test.go
index 4f32994587..e7a9243bc2 100644
--- a/notifier/notifier_test.go
+++ b/notifier/notifier_test.go
@@ -219,17 +219,19 @@ func TestHandlerSendAllRemapPerAm(t *testing.T) {
 		errc      = make(chan error, 1)
 		expected1 = make([]*Alert, 0, maxBatchSize)
 		expected2 = make([]*Alert, 0, maxBatchSize)
+		expected3 = make([]*Alert, 0)
 
-		status1, status2 atomic.Int32
+		statusOK atomic.Int32
 	)
-	status1.Store(int32(http.StatusOK))
-	status2.Store(int32(http.StatusOK))
+	statusOK.Store(int32(http.StatusOK))
 
-	server1 := newTestHTTPServerBuilder(&expected1, errc, "", "", &status1)
-	server2 := newTestHTTPServerBuilder(&expected2, errc, "", "", &status2)
+	server1 := newTestHTTPServerBuilder(&expected1, errc, "", "", &statusOK)
+	server2 := newTestHTTPServerBuilder(&expected2, errc, "", "", &statusOK)
+	server3 := newTestHTTPServerBuilder(&expected3, errc, "", "", &statusOK)
 
 	defer server1.Close()
 	defer server2.Close()
+	defer server3.Close()
 
 	h := NewManager(&Options{}, nil)
 	h.alertmanagers = make(map[string]*alertmanagerSet)
@@ -247,38 +249,68 @@ func TestHandlerSendAllRemapPerAm(t *testing.T) {
 		},
 	}
 
-	h.alertmanagers["1"] = &alertmanagerSet{
-		ams: []alertmanager{
-			alertmanagerMock{
-				urlf: func() string { return server1.URL },
-			},
+	am3Cfg := config.DefaultAlertmanagerConfig
+	am3Cfg.Timeout = model.Duration(time.Second)
+	am3Cfg.AlertRelabelConfigs = []*relabel.Config{
+		{
+			SourceLabels: model.LabelNames{"alertname"},
+			Action:       "drop",
+			Regex:        relabel.MustNewRegexp(".+"),
 		},
-		cfg: &am1Cfg,
 	}
 
-	h.alertmanagers["2"] = &alertmanagerSet{
-		ams: []alertmanager{
-			alertmanagerMock{
-				urlf: func() string { return server2.URL },
+	h.alertmanagers = map[string]*alertmanagerSet{
+		// Drop no alerts.
+		"1": {
+			ams: []alertmanager{
+				alertmanagerMock{
+					urlf: func() string { return server1.URL },
+				},
 			},
+			cfg: &am1Cfg,
+		},
+		// Drop only alerts with the "alertnamedrop" label.
+		"2": {
+			ams: []alertmanager{
+				alertmanagerMock{
+					urlf: func() string { return server2.URL },
+				},
+			},
+			cfg: &am2Cfg,
+		},
+		// Drop all alerts.
+		"3": {
+			ams: []alertmanager{
+				alertmanagerMock{
+					urlf: func() string { return server3.URL },
+				},
+			},
+			cfg: &am3Cfg,
+		},
+		// Empty list of Alertmanager endpoints.
+		"4": {
+			ams: []alertmanager{},
+			cfg: &config.DefaultAlertmanagerConfig,
 		},
-		cfg: &am2Cfg,
 	}
 
 	for i := range make([]struct{}, maxBatchSize/2) {
-		h.queue = append(h.queue, &Alert{
-			Labels: labels.FromStrings("alertname", fmt.Sprintf("%d", i)),
-		})
-		h.queue = append(h.queue, &Alert{
-			Labels: labels.FromStrings("alertnamedrop", fmt.Sprintf("%d", i)),
-		})
+		h.queue = append(h.queue,
+			&Alert{
+				Labels: labels.FromStrings("alertname", fmt.Sprintf("%d", i)),
+			},
+			&Alert{
+				Labels: labels.FromStrings("alertname", "test", "alertnamedrop", fmt.Sprintf("%d", i)),
+			},
+		)
 
-		expected1 = append(expected1, &Alert{
-			Labels: labels.FromStrings("alertname", fmt.Sprintf("%d", i)),
-		})
-		expected1 = append(expected1, &Alert{
-			Labels: labels.FromStrings("alertnamedrop", fmt.Sprintf("%d", i)),
-		})
+		expected1 = append(expected1,
+			&Alert{
+				Labels: labels.FromStrings("alertname", fmt.Sprintf("%d", i)),
+			}, &Alert{
+				Labels: labels.FromStrings("alertname", "test", "alertnamedrop", fmt.Sprintf("%d", i)),
+			},
+		)
 
 		expected2 = append(expected2, &Alert{
 			Labels: labels.FromStrings("alertname", fmt.Sprintf("%d", i)),
@@ -296,6 +328,13 @@ func TestHandlerSendAllRemapPerAm(t *testing.T) {
 
 	require.True(t, h.sendAll(h.queue...), "all sends failed unexpectedly")
 	checkNoErr()
+
+	// Verify that individual locks are released.
+	for k := range h.alertmanagers {
+		h.alertmanagers[k].mtx.Lock()
+		h.alertmanagers[k].ams = nil
+		h.alertmanagers[k].mtx.Unlock()
+	}
 }
 
 func TestCustomDo(t *testing.T) {
diff --git a/web/ui/module/codemirror-promql/package.json b/web/ui/module/codemirror-promql/package.json
index c364e05584..1e50b43b53 100644
--- a/web/ui/module/codemirror-promql/package.json
+++ b/web/ui/module/codemirror-promql/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@prometheus-io/codemirror-promql",
-  "version": "0.51.1",
+  "version": "0.51.2",
   "description": "a CodeMirror mode for the PromQL language",
   "types": "dist/esm/index.d.ts",
   "module": "dist/esm/index.js",
@@ -29,7 +29,7 @@
   },
   "homepage": "https://github.com/prometheus/prometheus/blob/main/web/ui/module/codemirror-promql/README.md",
   "dependencies": {
-    "@prometheus-io/lezer-promql": "0.51.1",
+    "@prometheus-io/lezer-promql": "0.51.2",
     "lru-cache": "^7.18.3"
   },
   "devDependencies": {
diff --git a/web/ui/module/lezer-promql/package.json b/web/ui/module/lezer-promql/package.json
index f5ea66602f..494458a47a 100644
--- a/web/ui/module/lezer-promql/package.json
+++ b/web/ui/module/lezer-promql/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@prometheus-io/lezer-promql",
-  "version": "0.51.1",
+  "version": "0.51.2",
   "description": "lezer-based PromQL grammar",
   "main": "dist/index.cjs",
   "type": "module",
diff --git a/web/ui/package-lock.json b/web/ui/package-lock.json
index fdb348d669..ecdbf18a11 100644
--- a/web/ui/package-lock.json
+++ b/web/ui/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "prometheus-io",
-  "version": "0.51.1",
+  "version": "0.51.2",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "prometheus-io",
-      "version": "0.51.1",
+      "version": "0.51.2",
       "workspaces": [
         "react-app",
         "module/*"
@@ -30,10 +30,10 @@
     },
     "module/codemirror-promql": {
       "name": "@prometheus-io/codemirror-promql",
-      "version": "0.51.1",
+      "version": "0.51.2",
       "license": "Apache-2.0",
       "dependencies": {
-        "@prometheus-io/lezer-promql": "0.51.1",
+        "@prometheus-io/lezer-promql": "0.51.2",
         "lru-cache": "^7.18.3"
       },
       "devDependencies": {
@@ -69,7 +69,7 @@
     },
     "module/lezer-promql": {
       "name": "@prometheus-io/lezer-promql",
-      "version": "0.51.1",
+      "version": "0.51.2",
       "license": "Apache-2.0",
       "devDependencies": {
         "@lezer/generator": "^1.5.1",
@@ -19233,7 +19233,7 @@
     },
     "react-app": {
       "name": "@prometheus-io/app",
-      "version": "0.51.1",
+      "version": "0.51.2",
       "dependencies": {
         "@codemirror/autocomplete": "^6.11.1",
         "@codemirror/commands": "^6.3.2",
@@ -19251,7 +19251,7 @@
         "@lezer/lr": "^1.3.14",
         "@nexucis/fuzzy": "^0.4.1",
         "@nexucis/kvsearch": "^0.8.1",
-        "@prometheus-io/codemirror-promql": "0.51.1",
+        "@prometheus-io/codemirror-promql": "0.51.2",
         "bootstrap": "^4.6.2",
         "css.escape": "^1.5.1",
         "downshift": "^7.6.2",
diff --git a/web/ui/package.json b/web/ui/package.json
index 0772aad7de..9c58f592c1 100644
--- a/web/ui/package.json
+++ b/web/ui/package.json
@@ -28,5 +28,5 @@
     "ts-jest": "^29.1.1",
     "typescript": "^4.9.5"
   },
-  "version": "0.51.1"
+  "version": "0.51.2"
 }
diff --git a/web/ui/react-app/package.json b/web/ui/react-app/package.json
index b47c9180a3..bd600720e2 100644
--- a/web/ui/react-app/package.json
+++ b/web/ui/react-app/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@prometheus-io/app",
-  "version": "0.51.1",
+  "version": "0.51.2",
   "private": true,
   "dependencies": {
     "@codemirror/autocomplete": "^6.11.1",
@@ -19,7 +19,7 @@
     "@lezer/lr": "^1.3.14",
     "@nexucis/fuzzy": "^0.4.1",
     "@nexucis/kvsearch": "^0.8.1",
-    "@prometheus-io/codemirror-promql": "0.51.1",
+    "@prometheus-io/codemirror-promql": "0.51.2",
     "bootstrap": "^4.6.2",
     "css.escape": "^1.5.1",
     "downshift": "^7.6.2",