diff --git a/pkg/compact/downsample/downsample.go b/pkg/compact/downsample/downsample.go
index 6989677b2a0..8b638d5c5cb 100644
--- a/pkg/compact/downsample/downsample.go
+++ b/pkg/compact/downsample/downsample.go
@@ -941,7 +941,7 @@ func downsampleAggrLoop(
 
 func expandChunkIterator(it chunkenc.Iterator, encoding chunkenc.Encoding, samples *[]sample) error {
 	switch encoding {
-	case chunkenc.EncXOR:
+	case chunkenc.EncXOR, chunkenc.EncXOR2:
 		return expandXorChunkIterator(it, samples)
 	case chunkenc.EncFloatHistogram:
 		return expandFloatHistogramChunkIterator(it, samples)
diff --git a/pkg/compact/downsample/downsample_test.go b/pkg/compact/downsample/downsample_test.go
index 180619af2b4..bcdca8917b1 100644
--- a/pkg/compact/downsample/downsample_test.go
+++ b/pkg/compact/downsample/downsample_test.go
@@ -1211,6 +1211,22 @@ func TestExpandChunkIterator(t *testing.T) {
 	testutil.Equals(t, []sample{{t: 100, v: 1}, {t: 200, v: 2}, {t: 200, v: 3}, {t: 201, v: 4}, {t: 300, v: 6}, {t: 500, v: 5}}, res)
 }
 
+func TestExpandChunkIteratorXOR2(t *testing.T) {
+	// XOR2 (chunkenc.EncXOR2) is the Prometheus 3.11+ float chunk encoding used to
+	// persist real start timestamps; it must expand identically to plain XOR.
+	var res []sample
+	testutil.Ok(t,
+		expandChunkIterator(
+			newSampleIterator([]sample{
+				{t: 100, v: 1}, {t: 200, v: 2}, {t: 200, v: 3}, {t: 201, v: 4}, {t: 200, v: 5},
+				{t: 300, v: 6}, {t: 400, v: math.Float64frombits(value.StaleNaN)}, {t: 500, v: 5},
+			}), chunkenc.EncXOR2, &res,
+		),
+	)
+
+	testutil.Equals(t, []sample{{t: 100, v: 1}, {t: 200, v: 2}, {t: 200, v: 3}, {t: 201, v: 4}, {t: 300, v: 6}, {t: 500, v: 5}}, res)
+}
+
 var (
 	// Decoded excerpt of pkg/query/testdata/issue2401-seriesresponses.json without overlaps (downsampling works directly on blocks).
 	realisticChkDataWithStaleMarker = [][]sample{
@@ -2881,4 +2897,7 @@ func TestDownsampleNHCutNewChunk(t *testing.T) {
 	require.False(t, cutNewChunk(chunkenc.EncFloatHistogram, chunkenc.EncFloatHistogram))
 	require.True(t, cutNewChunk(chunkenc.EncXOR, chunkenc.EncFloatHistogram))
 	require.True(t, cutNewChunk(chunkenc.EncXOR, chunkenc.EncHistogram))
+	require.True(t, cutNewChunk(chunkenc.EncXOR2, chunkenc.EncHistogram))
+	require.False(t, cutNewChunk(chunkenc.EncXOR2, chunkenc.EncXOR))
+	require.False(t, cutNewChunk(chunkenc.EncXOR, chunkenc.EncXOR2))
 }
diff --git a/pkg/query/iter.go b/pkg/query/iter.go
index 3736ba068bd..c1d0174d28e 100644
--- a/pkg/query/iter.go
+++ b/pkg/query/iter.go
@@ -188,6 +188,8 @@ func chunkEncoding(e storepb.Chunk_Encoding) chunkenc.Encoding {
 	switch e {
 	case storepb.Chunk_XOR:
 		return chunkenc.EncXOR
+	case storepb.Chunk_XOR2:
+		return chunkenc.EncXOR2
 	case storepb.Chunk_HISTOGRAM:
 		return chunkenc.EncHistogram
 	case storepb.Chunk_FLOAT_HISTOGRAM:
diff --git a/pkg/query/iter_test.go b/pkg/query/iter_test.go
new file mode 100644
index 00000000000..5177cd5b9b0
--- /dev/null
+++ b/pkg/query/iter_test.go
@@ -0,0 +1,20 @@
+// Copyright (c) The Thanos Authors.
+// Licensed under the Apache License 2.0.
+
+package query
+
+import (
+	"testing"
+
+	"github.com/efficientgo/core/testutil"
+	"github.com/prometheus/prometheus/tsdb/chunkenc"
+
+	"github.com/thanos-io/thanos/pkg/store/storepb"
+)
+
+func TestChunkEncoding_XOR2(t *testing.T) {
+	// XOR2 (chunkenc.EncXOR2) is the Prometheus 3.11+ float chunk encoding used to
+	// persist real start timestamps. The querier must be able to decode it back from
+	// the storepb wire encoding after a store-gateway serves it.
+	testutil.Equals(t, chunkenc.EncXOR2, chunkEncoding(storepb.Chunk_XOR2))
+}
diff --git a/pkg/store/bucket.go b/pkg/store/bucket.go
index 09f2e2492c2..a567beeb3fa 100644
--- a/pkg/store/bucket.go
+++ b/pkg/store/bucket.go
@@ -1491,6 +1491,8 @@ func chunkToStoreEncoding(in chunkenc.Encoding) storepb.Chunk_Encoding {
 	switch in {
 	case chunkenc.EncXOR:
 		return storepb.Chunk_XOR
+	case chunkenc.EncXOR2:
+		return storepb.Chunk_XOR2
 	case chunkenc.EncHistogram:
 		return storepb.Chunk_HISTOGRAM
 	case chunkenc.EncFloatHistogram:
diff --git a/pkg/store/bucket_test.go b/pkg/store/bucket_test.go
index 8b8d9f58bca..e1cac733c7b 100644
--- a/pkg/store/bucket_test.go
+++ b/pkg/store/bucket_test.go
@@ -77,6 +77,12 @@ func TestRawChunkReset(t *testing.T) {
 	testutil.Equals(t, []byte(r), []byte{3, 4})
 }
 
+func TestChunkToStoreEncoding_XOR2(t *testing.T) {
+	// XOR2 (chunkenc.EncXOR2) is the Prometheus 3.11+ float chunk encoding used to
+	// persist real start timestamps. Serving it via the store-gateway must not panic.
+	testutil.Equals(t, storepb.Chunk_XOR2, chunkToStoreEncoding(chunkenc.EncXOR2))
+}
+
 func TestBucketBlock_Property(t *testing.T) {
 	t.Parallel()
 
diff --git a/pkg/store/storepb/custom.go b/pkg/store/storepb/custom.go
index ff86a855dc8..1dd81e13f4e 100644
--- a/pkg/store/storepb/custom.go
+++ b/pkg/store/storepb/custom.go
@@ -501,9 +501,9 @@ func LabelsToPromLabelsUnsafe(lset []Label) labels.Labels {
 	return labelpb.ZLabelsToPromLabels(lset)
 }
 
-// XORNumSamples return number of samples. Returns 0 if it's not XOR chunk.
+// XORNumSamples return number of samples. Returns 0 if it's not an XOR or XOR2 chunk.
 func (m *Chunk) XORNumSamples() int {
-	if m.Type == Chunk_XOR {
+	if m.Type == Chunk_XOR || m.Type == Chunk_XOR2 {
 		return int(binary.BigEndian.Uint16(m.Data))
 	}
 	return 0
diff --git a/pkg/store/storepb/custom_test.go b/pkg/store/storepb/custom_test.go
index bc94bc0e235..b95759fe563 100644
--- a/pkg/store/storepb/custom_test.go
+++ b/pkg/store/storepb/custom_test.go
@@ -4,6 +4,7 @@
 package storepb
 
 import (
+	"encoding/binary"
 	"fmt"
 	"path/filepath"
 	"sort"
@@ -25,6 +26,17 @@ type sample struct {
 	v float64
 }
 
+func TestChunk_XORNumSamples_XOR2(t *testing.T) {
+	// XOR2 (Chunk_XOR2) is the Prometheus 3.11+ float chunk encoding used to persist
+	// real start timestamps; it shares XOR's wire header (sample count as first 2 bytes)
+	// and must be counted the same way for series stats.
+	data := make([]byte, 2)
+	binary.BigEndian.PutUint16(data, 7)
+	c := &Chunk{Type: Chunk_XOR2, Data: data}
+
+	testutil.Equals(t, 7, c.XORNumSamples())
+}
+
 type listSeriesSet struct {
 	series []Series
 	idx    int
diff --git a/pkg/store/storepb/types.pb.go b/pkg/store/storepb/types.pb.go
index 13e81010764..efc19037cd6 100644
--- a/pkg/store/storepb/types.pb.go
+++ b/pkg/store/storepb/types.pb.go
@@ -66,18 +66,21 @@ const (
 	Chunk_XOR             Chunk_Encoding = 0
 	Chunk_HISTOGRAM       Chunk_Encoding = 1
 	Chunk_FLOAT_HISTOGRAM Chunk_Encoding = 2
+	Chunk_XOR2            Chunk_Encoding = 3
 )
 
 var Chunk_Encoding_name = map[int32]string{
 	0: "XOR",
 	1: "HISTOGRAM",
 	2: "FLOAT_HISTOGRAM",
+	3: "XOR2",
 }
 
 var Chunk_Encoding_value = map[string]int32{
 	"XOR":             0,
 	"HISTOGRAM":       1,
 	"FLOAT_HISTOGRAM": 2,
+	"XOR2":            3,
 }
 
 func (x Chunk_Encoding) String() string {
@@ -331,44 +334,45 @@ func init() {
 func init() { proto.RegisterFile("store/storepb/types.proto", fileDescriptor_121fba57de02d8e0) }
 
 var fileDescriptor_121fba57de02d8e0 = []byte{
-	// 588 bytes of a gzipped FileDescriptorProto
+	// 595 bytes of a gzipped FileDescriptorProto
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-	0x00, 0x00, 0xff, 0xff, 0xa9, 0xae, 0xe0, 0xc2, 0xd2, 0x03, 0x00, 0x00,
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+	0xa3, 0x0f, 0x34, 0x13, 0x11, 0x1d, 0x13, 0xc6, 0x27, 0x69, 0xc2, 0xd9, 0x89, 0xc8, 0xa8, 0x60,
+	0xe1, 0x95, 0x1c, 0xd3, 0x8f, 0x6d, 0x72, 0x64, 0x6b, 0xb8, 0x0e, 0x95, 0x76, 0xa7, 0x47, 0xfa,
+	0x36, 0xea, 0x3c, 0x99, 0xfd, 0x71, 0xb5, 0xd9, 0xad, 0x8b, 0xae, 0x6f, 0x5d, 0xf4, 0xfb, 0xd6,
+	0x45, 0x5f, 0xe6, 0xae, 0x76, 0x3d, 0x77, 0xb5, 0x9f, 0x73, 0x57, 0x3b, 0xab, 0x96, 0x8f, 0x78,
+	0x60, 0xa9, 0x67, 0xf8, 0xfc, 0x6f, 0x00, 0x00, 0x00, 0xff, 0xff, 0x6e, 0x72, 0xe4, 0x16, 0xdc,
+	0x03, 0x00, 0x00,
 }
 
 func (m *Chunk) Marshal() (dAtA []byte, err error) {
diff --git a/pkg/store/storepb/types.proto b/pkg/store/storepb/types.proto
index c998fec9246..3a9cec71447 100644
--- a/pkg/store/storepb/types.proto
+++ b/pkg/store/storepb/types.proto
@@ -25,6 +25,7 @@ message Chunk {
     XOR = 0;
     HISTOGRAM = 1;
     FLOAT_HISTOGRAM = 2;
+    XOR2 = 3;
   }
   Encoding type  = 1;
   bytes data     = 2;