diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index b32eabd4d6..e3969e1fe8 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -37,6 +37,7 @@ The sandbox cluster installation is running under the constant load generated by
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): prevent from `FATAL: cannot flush metainfo` panic when [`-remoteWrite.multitenantURL`](https://docs.victoriametrics.com/vmagent.html#multitenancy) command-line flag is set. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5357).
 
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): properly decode zstd-encoded data blocks received via [VictoriaMetrics remote_write protocol](https://docs.victoriametrics.com/vmagent.html#victoriametrics-remote-write-protocol). See [this issue comment](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5301#issuecomment-1815871992).
+* BUGFIX:  [vmagent](https://docs.victoriametrics.com/vmagent.html): properly add new labels at `output_relabel_configs` during [stream aggregation](https://docs.victoriametrics.com/stream-aggregation.html). Previously this could lead to corrupted labels in output samples. Thanks to @ChengChung for providing [detailed report for this bug](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5402).
 
 
 ## [v1.95.1](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.95.1)
diff --git a/lib/promrelabel/relabel.go b/lib/promrelabel/relabel.go
index e2d5d68da1..f1e0a6ce75 100644
--- a/lib/promrelabel/relabel.go
+++ b/lib/promrelabel/relabel.go
@@ -70,11 +70,15 @@ func (prc *parsedRelabelConfig) String() string {
 //
 // It returns DebugStep list - one entry per each applied relabeling step.
 func (pcs *ParsedConfigs) ApplyDebug(labels []prompbmarshal.Label) ([]prompbmarshal.Label, []DebugStep) {
-	labels, dss := pcs.applyInternal(labels, 0, true)
+	// Protect from overwriting labels between len(labels) and cap(labels) by limiting labels capacity to its length.
+	labels, dss := pcs.applyInternal(labels[:len(labels):len(labels)], 0, true)
 	return labels, dss
 }
 
 // Apply applies pcs to labels starting from the labelsOffset.
+//
+// Apply() may add additional labels after the len(labels), so make sure it doesn't corrupt in-use labels
+// stored between len(labels) and cap(labels).
 func (pcs *ParsedConfigs) Apply(labels []prompbmarshal.Label, labelsOffset int) []prompbmarshal.Label {
 	labels, _ = pcs.applyInternal(labels, labelsOffset, false)
 	return labels
diff --git a/lib/streamaggr/streamaggr.go b/lib/streamaggr/streamaggr.go
index 705709dc7f..c3d7bacddb 100644
--- a/lib/streamaggr/streamaggr.go
+++ b/lib/streamaggr/streamaggr.go
@@ -471,22 +471,30 @@ func (a *aggregator) flush() {
 
 		tss := ctx.tss
 
-		// Apply output relabeling
-		if a.outputRelabeling != nil {
-			dst := tss[:0]
-			for _, ts := range tss {
-				ts.Labels = a.outputRelabeling.Apply(ts.Labels, 0)
-				if len(ts.Labels) == 0 {
-					// The metric has been deleted by the relabeling
-					continue
-				}
-				dst = append(dst, ts)
-			}
-			tss = dst
+		if a.outputRelabeling == nil {
+			// Fast path - push the output metrics.
+			a.pushFunc(tss)
+			continue
 		}
 
-		// Push the output metrics.
-		a.pushFunc(tss)
+		// Slower path - apply output relabeling and then push the output metrics.
+		auxLabels := promutils.GetLabels()
+		dstLabels := auxLabels.Labels[:0]
+		dst := tss[:0]
+		for _, ts := range tss {
+			dstLabelsLen := len(dstLabels)
+			dstLabels = append(dstLabels, ts.Labels...)
+			dstLabels = a.outputRelabeling.Apply(dstLabels, dstLabelsLen)
+			if len(dstLabels) == dstLabelsLen {
+				// The metric has been deleted by the relabeling
+				continue
+			}
+			ts.Labels = dstLabels[dstLabelsLen:]
+			dst = append(dst, ts)
+		}
+		a.pushFunc(dst)
+		auxLabels.Labels = dstLabels
+		promutils.PutLabels(auxLabels)
 	}
 }
 
diff --git a/lib/streamaggr/streamaggr_test.go b/lib/streamaggr/streamaggr_test.go
index 91ced47e1c..33924c8e21 100644
--- a/lib/streamaggr/streamaggr_test.go
+++ b/lib/streamaggr/streamaggr_test.go
@@ -686,6 +686,35 @@ cpu_usage{cpu="2"} 90
 cpu_usage:1m_without_cpu_quantiles{quantile="0.5"} 13.3
 cpu_usage:1m_without_cpu_quantiles{quantile="1"} 90
 `, "1111111")
+
+	// append additional label
+	f(`
+- interval: 1m
+  without: [abc]
+  outputs: [count_samples, sum_samples, count_series]
+  output_relabel_configs:
+  - action: replace_all
+    source_labels: [__name__]
+    regex: ":|_"
+    replacement: "-"
+    target_label: __name__
+  - action: drop
+    source_labels: [de]
+    regex: fg
+  - target_label: new_label
+    replacement: must_keep_metric_name
+`, `
+foo{abc="123"} 4
+bar 5
+foo{abc="123"} 8.5
+foo{abc="456",de="fg"} 8
+`, `bar-1m-without-abc-count-samples{new_label="must_keep_metric_name"} 1
+bar-1m-without-abc-count-series{new_label="must_keep_metric_name"} 1
+bar-1m-without-abc-sum-samples{new_label="must_keep_metric_name"} 5
+foo-1m-without-abc-count-samples{new_label="must_keep_metric_name"} 2
+foo-1m-without-abc-count-series{new_label="must_keep_metric_name"} 1
+foo-1m-without-abc-sum-samples{new_label="must_keep_metric_name"} 12.5
+`, "1111")
 }
 
 func TestAggregatorsWithDedupInterval(t *testing.T) {