diff --git a/notify/impl.go b/notify/impl.go
index 23ae8637..765556cb 100644
--- a/notify/impl.go
+++ b/notify/impl.go
@@ -585,9 +585,10 @@ func (n *Slack) Notify(ctx context.Context, as ...*types.Alert) (bool, error) {
 		return false, err
 	}
 
-	resp, err := post(ctx, c, n.conf.APIURL.String(), contentTypeJSON, &buf)
+	u := n.conf.APIURL.String()
+	resp, err := post(ctx, c, u, contentTypeJSON, &buf)
 	if err != nil {
-		return true, err
+		return true, redactURL(err)
 	}
 	resp.Body.Close()
 
@@ -1276,6 +1277,16 @@ func hashKey(s string) string {
 	return fmt.Sprintf("%x", h.Sum(nil))
 }
 
+// redactURL removes the URL part from an error of *url.Error type.
+func redactURL(err error) error {
+	e, ok := err.(*url.Error)
+	if !ok {
+		return err
+	}
+	e.URL = "<redacted>"
+	return e
+}
+
 func post(ctx context.Context, client *http.Client, url string, bodyType string, body io.Reader) (*http.Response, error) {
 	req, err := http.NewRequest("POST", url, body)
 	if err != nil {
diff --git a/notify/impl_test.go b/notify/impl_test.go
index 1ebafb12..a731cb03 100644
--- a/notify/impl_test.go
+++ b/notify/impl_test.go
@@ -21,17 +21,19 @@ import (
 	"io"
 	"io/ioutil"
 	"net/http"
+	"net/http/httptest"
 	"net/url"
 	"testing"
 	"time"
 
 	"github.com/go-kit/kit/log"
+	commoncfg "github.com/prometheus/common/config"
+	"github.com/prometheus/common/model"
 	"github.com/stretchr/testify/require"
 
 	"github.com/prometheus/alertmanager/config"
 	"github.com/prometheus/alertmanager/template"
 	"github.com/prometheus/alertmanager/types"
-	"github.com/prometheus/common/model"
 )
 
 func TestWebhookRetry(t *testing.T) {
@@ -122,6 +124,37 @@ func TestSlackRetry(t *testing.T) {
 	}
 }
 
+func TestSlackRedactedURL(t *testing.T) {
+	done := make(chan struct{})
+	ctx, cancel := context.WithCancel(context.Background())
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		cancel()
+		<-done
+	}))
+	defer func() {
+		close(done)
+		srv.Close()
+	}()
+
+	u, err := url.Parse(srv.URL)
+	require.NoError(t, err)
+
+	notifier := NewSlack(
+		&config.SlackConfig{
+			APIURL:     &config.SecretURL{URL: u},
+			HTTPConfig: &commoncfg.HTTPClientConfig{},
+		},
+		createTmpl(t),
+		log.NewNopLogger(),
+	)
+
+	ok, err := notifier.Notify(ctx, []*types.Alert{}...)
+	require.True(t, ok)
+	require.Error(t, err)
+	require.NotContains(t, err.Error(), srv.URL)
+}
+
 func TestHipchatRetry(t *testing.T) {
 	notifier := new(Hipchat)
 	retryCodes := append(defaultRetryCodes(), http.StatusTooManyRequests)