diff --git a/pkg/store/storepb/custom.go b/pkg/store/storepb/custom.go
index 6c806cb2b4..efe711d593 100644
--- a/pkg/store/storepb/custom.go
+++ b/pkg/store/storepb/custom.go
@@ -497,32 +497,45 @@ func (m *SeriesResponse) Marshal() (dAtA []byte, err error) {
 
 	var respBuf []byte
 
-	// No pool defined, allocate directly.
+	// Slow path with no sync.Pool.
 	if m.respPool == nil {
 		respBuf = make([]byte, size)
-	} else {
-		if m.respBuf == nil {
-			poolBuf := m.respPool.Get()
-			if poolBuf == nil {
-				respBuf = make([]byte, size)
-				m.respBuf = &respBuf
-			} else {
-				m.respBuf = poolBuf.(*[]byte)
-				respBuf = *m.respBuf
-			}
+
+		n, err := m.MarshalToSizedBuffer(respBuf)
+		if err != nil {
+			return nil, err
+		}
+		return respBuf[len(respBuf)-n:], nil
+	}
+
+	// Fast path with sync.Pool.
+	// m.respBuf must not be nil so that it would be returned to the pool.
+
+	// If no pre-allocated buffer has been passed then try to get a new one.
+	if m.respBuf == nil {
+		poolBuf := m.respPool.Get()
+		// No previous buffer found in the pool, try to allocate.
+		if poolBuf == nil {
+			respBuf = make([]byte, size)
 		} else {
-			if cap(*m.respBuf) < size {
-				if m.respPool != nil {
-					m.respPool.Put(m.respBuf)
-				}
-				respBuf = make([]byte, size)
-				m.respBuf = &respBuf
-			} else {
-				respBuf = *m.respBuf
-			}
+			// Found something, let's see if it is big enough.
+			respBuf = *(poolBuf.(*[]byte))
 		}
+	} else {
+		respBuf = *m.respBuf
+	}
+
+	// Last sanity check of the size before the marshaling.
+	if cap(respBuf) < size {
+		if m.respPool != nil {
+			m.respPool.Put(&respBuf)
+		}
+		respBuf = make([]byte, size)
 	}
+	m.respBuf = &respBuf
 
+	// Possibly trim it so that there wouldn't be left-over "garbage" in the slice.
+	// TODO: check if it is needed to always trim this.
 	marshalBuf := respBuf[:size]
 	n, err := m.MarshalToSizedBuffer(marshalBuf)
 	if err != nil {
diff --git a/pkg/store/storepb/custom_test.go b/pkg/store/storepb/custom_test.go
index 35901d52bc..db4d34f2d0 100644
--- a/pkg/store/storepb/custom_test.go
+++ b/pkg/store/storepb/custom_test.go
@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"path/filepath"
 	"sort"
+	"sync"
 	"testing"
 
 	"github.com/pkg/errors"
@@ -525,3 +526,47 @@ func TestMatchersToString_Translate(t *testing.T) {
 
 	}
 }
+
+// Tests whether the Marshal() function properly checks the size
+// of a slice returned from a sync.Pool.
+// Regression test against https://github.com/thanos-io/thanos/issues/4591.
+func TestMarshalChecksSize(t *testing.T) {
+	var s Series
+
+	rawData := []rawSeries{
+		{
+			lset: labels.Labels{labels.Label{Name: "a", Value: "c"}},
+			chunks: [][]sample{
+				{{t: 11, v: 11}, {t: 12, v: 12}, {t: 13, v: 13}, {t: 14, v: 14}},
+				{{t: 1, v: 1}, {t: 2, v: 2}, {t: 3, v: 3}, {t: 4, v: 4}},
+				{{t: 20, v: 20}, {t: 21, v: 21}, {t: 22, v: 22}, {t: 24, v: 24}},
+				{{t: 11, v: 11}, {t: 12, v: 12}, {t: 13, v: 13}, {t: 14, v: 14}},
+				{{t: 15, v: 15}, {t: 16, v: 16}, {t: 17, v: 17}, {t: 18, v: 18}},
+				{{t: 20, v: 20}, {t: 21, v: 21}, {t: 22, v: 23}, {t: 24, v: 24}},
+				{{t: 11, v: 11}, {t: 12, v: 12}, {t: 13, v: 13}, {t: 14, v: 14}},
+			},
+		},
+	}
+
+	listSS := newListSeriesSet(t, rawData)
+	lset, chks := listSS.At()
+
+	s.Chunks = chks
+	s.Labels = labelpb.ZLabelsFromPromLabels(lset)
+
+	resp := NewSeriesResponse(&s)
+
+	smallPool := sync.Pool{
+		New: func() interface{} {
+			b := make([]byte, 1)
+			return &b
+		},
+	}
+	resp.respPool = &smallPool
+
+	d, err := resp.Marshal()
+	testutil.Ok(t, err)
+	testutil.Assert(t, d != nil)
+
+	resp.Close()
+}