diff --git a/pkg/services/ngalert/sender/router.go b/pkg/services/ngalert/sender/router.go
index 077901d9613..a1e20abd80c 100644
--- a/pkg/services/ngalert/sender/router.go
+++ b/pkg/services/ngalert/sender/router.go
@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"net/url"
+	"path"
 	"sort"
 	"sync"
 	"time"
@@ -270,7 +271,10 @@ func (d *AlertsRouter) buildExternalURL(ds *datasources.DataSource) (string, err
 			if parsed.Path == "" {
 				parsed.Path = "/"
 			}
-			parsed = parsed.JoinPath("/alertmanager")
+			lastSegment := path.Base(parsed.Path)
+			if lastSegment != "alertmanager" {
+				parsed = parsed.JoinPath("/alertmanager")
+			}
 		default:
 		}
 	}
diff --git a/pkg/services/ngalert/sender/router_test.go b/pkg/services/ngalert/sender/router_test.go
index 361d738191f..20cce2e4691 100644
--- a/pkg/services/ngalert/sender/router_test.go
+++ b/pkg/services/ngalert/sender/router_test.go
@@ -540,6 +540,42 @@ func TestBuildExternalURL(t *testing.T) {
 			},
 			expectedURL: "https://localhost:9000/path/to/am",
 		},
+		{
+			name: "do not add /alertmanager to path when last segment already contains it",
+			ds: &datasources.DataSource{
+				URL: "https://localhost:9000/path/to/alertmanager",
+				JsonData: func() *simplejson.Json {
+					r := simplejson.New()
+					r.Set("implementation", "mimir")
+					return r
+				}(),
+			},
+			expectedURL: "https://localhost:9000/path/to/alertmanager",
+		},
+		{
+			name: "add /alertmanager to path when last segment does not exactly match",
+			ds: &datasources.DataSource{
+				URL: "https://localhost:9000/path/to/alertmanagerasdf",
+				JsonData: func() *simplejson.Json {
+					r := simplejson.New()
+					r.Set("implementation", "mimir")
+					return r
+				}(),
+			},
+			expectedURL: "https://localhost:9000/path/to/alertmanagerasdf/alertmanager",
+		},
+		{
+			name: "add /alertmanager to path when exists but is not last segment",
+			ds: &datasources.DataSource{
+				URL: "https://localhost:9000/alertmanager/path/to/am",
+				JsonData: func() *simplejson.Json {
+					r := simplejson.New()
+					r.Set("implementation", "mimir")
+					return r
+				}(),
+			},
+			expectedURL: "https://localhost:9000/alertmanager/path/to/am/alertmanager",
+		},
 	}
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {