diff --git a/notify/slack/slack.go b/notify/slack/slack.go
index 22bb30932e..1a04a00f1a 100644
--- a/notify/slack/slack.go
+++ b/notify/slack/slack.go
@@ -18,6 +18,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"net/http"
 	"os"
 	"strings"
@@ -43,6 +44,8 @@ type Notifier struct {
 	logger  log.Logger
 	client  *http.Client
 	retrier *notify.Retrier
+
+	postJSONFunc func(ctx context.Context, client *http.Client, url string, body io.Reader) (*http.Response, error)
 }
 
 // New returns a new Slack notification handler.
@@ -53,11 +56,12 @@ func New(c *config.SlackConfig, t *template.Template, l log.Logger, httpOpts ...
 	}
 
 	return &Notifier{
-		conf:    c,
-		tmpl:    t,
-		logger:  l,
-		client:  client,
-		retrier: &notify.Retrier{},
+		conf:         c,
+		tmpl:         t,
+		logger:       l,
+		client:       client,
+		retrier:      &notify.Retrier{},
+		postJSONFunc: notify.PostJSON,
 	}, nil
 }
 
@@ -202,7 +206,7 @@ func (n *Notifier) Notify(ctx context.Context, as ...*types.Alert) (bool, error)
 		u = strings.TrimSpace(string(content))
 	}
 
-	resp, err := notify.PostJSON(ctx, n.client, u, &buf)
+	resp, err := n.postJSONFunc(ctx, n.client, u, &buf)
 	if err != nil {
 		return true, notify.RedactURL(err)
 	}
@@ -213,5 +217,7 @@ func (n *Notifier) Notify(ctx context.Context, as ...*types.Alert) (bool, error)
 	// https://api.slack.com/changelog/2016-05-17-changes-to-errors-for-incoming-webhooks
 	retry, err := n.retrier.Check(resp.StatusCode, resp.Body)
 	err = errors.Wrap(err, fmt.Sprintf("channel %q", req.Channel))
-	return retry, err
+	reasonErr := notify.NewErrorWithReason(notify.GetFailureReasonFromStatusCode(resp.StatusCode), err)
+
+	return retry, reasonErr
 }
diff --git a/notify/slack/slack_test.go b/notify/slack/slack_test.go
index 9a4f16cf0a..2329043da5 100644
--- a/notify/slack/slack_test.go
+++ b/notify/slack/slack_test.go
@@ -14,16 +14,25 @@
 package slack
 
 import (
+	"context"
 	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
 	"os"
 	"testing"
+	"time"
 
 	"github.com/go-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/notify"
 	"github.com/prometheus/alertmanager/notify/test"
+	"github.com/prometheus/alertmanager/types"
 )
 
 func TestSlackRetry(t *testing.T) {
@@ -102,3 +111,61 @@ func TestTrimmingSlackURLFromFile(t *testing.T) {
 
 	test.AssertNotifyLeaksNoSecret(ctx, t, notifier, u.String())
 }
+
+func TestNotifier_Notify_WithReason(t *testing.T) {
+	tests := []struct {
+		name           string
+		statusCode     int
+		expectedReason notify.Reason
+	}{
+		{
+			name:           "with a 4xx status code",
+			statusCode:     http.StatusUnauthorized,
+			expectedReason: notify.ClientErrorReason,
+		},
+		{
+			name:           "with a 5xx status code",
+			statusCode:     http.StatusInternalServerError,
+			expectedReason: notify.ServerErrorReason,
+		},
+		{
+			name:           "with any other status code",
+			statusCode:     http.StatusTemporaryRedirect,
+			expectedReason: notify.DefaultReason,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			apiurl, _ := url.Parse("https://slack.com/post.Message")
+			notifier, err := New(
+				&config.SlackConfig{
+					NotifierConfig: config.NotifierConfig{},
+					HTTPConfig:     &commoncfg.HTTPClientConfig{},
+					APIURL:         &config.SecretURL{URL: apiurl},
+				},
+				test.CreateTmpl(t),
+				log.NewNopLogger(),
+			)
+			require.NoError(t, err)
+
+			notifier.postJSONFunc = func(ctx context.Context, client *http.Client, url string, body io.Reader) (*http.Response, error) {
+				resp := httptest.NewRecorder()
+				resp.WriteHeader(tt.statusCode)
+				return resp.Result(), nil
+			}
+			ctx := context.Background()
+			ctx = notify.WithGroupKey(ctx, "1")
+
+			alert1 := &types.Alert{
+				Alert: model.Alert{
+					StartsAt: time.Now(),
+					EndsAt:   time.Now().Add(time.Hour),
+				},
+			}
+			_, err = notifier.Notify(ctx, alert1)
+			reasonError, ok := err.(*notify.ErrorWithReason)
+			require.True(t, ok)
+			require.Equal(t, tt.expectedReason, reasonError.Reason)
+		})
+	}
+}