diff --git a/CHANGELOG.md b/CHANGELOG.md
index 28f46064f5..22de8ef268 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -45,6 +45,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 
 ### Changed
 
+- [#4939](https://github.com/thanos-io/thanos/pull/4939) Sidecar: set Sidecar to NOT READY when it cannot establish a connection with Prometheus
 - [#4864](https://github.com/thanos-io/thanos/pull/4864) UI: Remove the old PromQL editor
 
 ## [v0.23.1](https://github.com/thanos-io/thanos/tree/release-0.23) - 2021.10.1
diff --git a/cmd/thanos/sidecar.go b/cmd/thanos/sidecar.go
index 56e0076f5d..291782e9a3 100644
--- a/cmd/thanos/sidecar.go
+++ b/cmd/thanos/sidecar.go
@@ -223,8 +223,10 @@ func runSidecar(
 				if err := m.UpdateLabels(iterCtx); err != nil {
 					level.Warn(logger).Log("msg", "heartbeat failed", "err", err)
 					promUp.Set(0)
+					statusProber.NotReady(err)
 				} else {
 					promUp.Set(1)
+					statusProber.Ready()
 				}
 
 				return nil
diff --git a/test/e2e/query_test.go b/test/e2e/query_test.go
index cd61e991ab..7f5e4b0f37 100644
--- a/test/e2e/query_test.go
+++ b/test/e2e/query_test.go
@@ -6,6 +6,7 @@ package e2e_test
 import (
 	"context"
 	"fmt"
+	"net/http"
 	"net/http/httptest"
 	"net/url"
 	"os"
@@ -103,6 +104,40 @@ func sortResults(res model.Vector) {
 	})
 }
 
+func TestSidecarNotReady(t *testing.T) {
+	t.Parallel()
+
+	e, err := e2e.NewDockerEnvironment("e2e_test_query")
+	testutil.Ok(t, err)
+	t.Cleanup(e2ethanos.CleanScenario(t, e))
+
+	prom, sidecar, err := e2ethanos.NewPrometheusWithSidecar(e, "alone", defaultPromConfig("prom-alone", 0, "", ""), "", e2ethanos.DefaultPrometheusImage())
+	testutil.Ok(t, err)
+	testutil.Ok(t, e2e.StartAndWaitReady(prom, sidecar))
+	testutil.Ok(t, prom.Stop())
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	// Sidecar should not be ready - it cannot accept traffic if Prometheus is down.
+	testutil.Ok(t, runutil.Retry(1*time.Second, ctx.Done(), func() (rerr error) {
+		req, err := http.NewRequestWithContext(ctx, "GET", "http://"+sidecar.Endpoint("http")+"/-/ready", nil)
+		if err != nil {
+			return err
+		}
+		resp, err := http.DefaultClient.Do(req)
+		if err != nil {
+			return err
+		}
+		defer runutil.CloseWithErrCapture(&rerr, resp.Body, "closing resp body")
+
+		if resp.StatusCode == 200 {
+			return fmt.Errorf("got status code %d", resp.StatusCode)
+		}
+		return nil
+	}))
+}
+
 func TestQuery(t *testing.T) {
 	t.Parallel()