diff --git a/tsdb/head.go b/tsdb/head.go
index 5972a9c5d..30ad8139a 100644
--- a/tsdb/head.go
+++ b/tsdb/head.go
@@ -1759,12 +1759,12 @@ type seriesHashmap struct {
 
 func (m *seriesHashmap) get(hash uint64, lset labels.Labels) *memSeries {
 	if s, found := m.unique[hash]; found {
-		if labels.Equal(s.lset, lset) {
+		if labels.Equal(s.labels(), lset) {
 			return s
 		}
 	}
 	for _, s := range m.conflicts[hash] {
-		if labels.Equal(s.lset, lset) {
+		if labels.Equal(s.labels(), lset) {
 			return s
 		}
 	}
@@ -1772,7 +1772,7 @@ func (m *seriesHashmap) get(hash uint64, lset labels.Labels) *memSeries {
 }
 
 func (m *seriesHashmap) set(hash uint64, s *memSeries) {
-	if existing, found := m.unique[hash]; !found || labels.Equal(existing.lset, s.lset) {
+	if existing, found := m.unique[hash]; !found || labels.Equal(existing.labels(), s.labels()) {
 		m.unique[hash] = s
 		return
 	}
@@ -1781,7 +1781,7 @@ func (m *seriesHashmap) set(hash uint64, s *memSeries) {
 	}
 	l := m.conflicts[hash]
 	for i, prev := range l {
-		if labels.Equal(prev.lset, s.lset) {
+		if labels.Equal(prev.labels(), s.labels()) {
 			l[i] = s
 			return
 		}
@@ -1931,7 +1931,7 @@ func (s *stripeSeries) gc(mint int64, minOOOMmapRef chunks.ChunkDiskMapperRef) (
 		series.lset.Range(func(l labels.Label) { affected[l] = struct{}{} })
 		s.hashes[hashShard].del(hash, series.ref)
 		delete(s.series[refShard], series.ref)
-		deletedForCallback[series.ref] = series.lset
+		deletedForCallback[series.ref] = series.lset // OK to access lset; series is locked at the top of this function.
 	}
 
 	s.iterForDeletion(check)
@@ -2023,7 +2023,7 @@ func (s *stripeSeries) getOrSet(hash uint64, lset labels.Labels, createSeries fu
 	}
 	// Setting the series in the s.hashes marks the creation of series
 	// as any further calls to this methods would return that series.
-	s.seriesLifecycleCallback.PostCreation(series.lset)
+	s.seriesLifecycleCallback.PostCreation(series.labels())
 
 	i = uint64(series.ref) & uint64(s.size-1)
 
@@ -2064,16 +2064,19 @@ func (s sample) Type() chunkenc.ValueType {
 // memSeries is the in-memory representation of a series. None of its methods
 // are goroutine safe and it is the caller's responsibility to lock it.
 type memSeries struct {
-	sync.Mutex
-
+	// Members up to the Mutex are not changed after construction, so can be accessed without a lock.
 	ref  chunks.HeadSeriesRef
-	lset labels.Labels
 	meta *metadata.Metadata
 
 	// Series labels hash to use for sharding purposes. The value is always 0 when sharding has not
 	// been explicitly enabled in TSDB.
 	shardHash uint64
 
+	// Everything after here should only be accessed with the lock held.
+	sync.Mutex
+
+	lset labels.Labels // Locking required with -tags dedupelabels, not otherwise.
+
 	// Immutable chunks on disk that have not yet gone into a block, in order of ascending time stamps.
 	// When compaction runs, chunks get moved into a block and all pointers are shifted like so:
 	//
diff --git a/tsdb/head_append.go b/tsdb/head_append.go
index 62c3727e2..f45ab606b 100644
--- a/tsdb/head_append.go
+++ b/tsdb/head_append.go
@@ -554,7 +554,7 @@ func (a *headAppender) AppendExemplar(ref storage.SeriesRef, lset labels.Labels,
 	// Ensure no empty labels have gotten through.
 	e.Labels = e.Labels.WithoutEmpty()
 
-	err := a.head.exemplars.ValidateExemplar(s.lset, e)
+	err := a.head.exemplars.ValidateExemplar(s.labels(), e)
 	if err != nil {
 		if errors.Is(err, storage.ErrDuplicateExemplar) || errors.Is(err, storage.ErrExemplarsDisabled) {
 			// Duplicate, don't return an error but don't accept the exemplar.
@@ -708,7 +708,7 @@ func (a *headAppender) GetRef(lset labels.Labels, hash uint64) (storage.SeriesRe
 		return 0, labels.EmptyLabels()
 	}
 	// returned labels must be suitable to pass to Append()
-	return storage.SeriesRef(s.ref), s.lset
+	return storage.SeriesRef(s.ref), s.labels()
 }
 
 // log writes all headAppender's data to the WAL.
@@ -816,7 +816,7 @@ func (a *headAppender) Commit() (err error) {
 			continue
 		}
 		// We don't instrument exemplar appends here, all is instrumented by storage.
-		if err := a.head.exemplars.AddExemplar(s.lset, e.exemplar); err != nil {
+		if err := a.head.exemplars.AddExemplar(s.labels(), e.exemplar); err != nil {
 			if errors.Is(err, storage.ErrOutOfOrderExemplar) {
 				continue
 			}
diff --git a/tsdb/head_dedupelabels.go b/tsdb/head_dedupelabels.go
new file mode 100644
index 000000000..203f92e6a
--- /dev/null
+++ b/tsdb/head_dedupelabels.go
@@ -0,0 +1,27 @@
+// Copyright 2024 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build dedupelabels
+
+package tsdb
+
+import (
+	"github.com/prometheus/prometheus/model/labels"
+)
+
+// Helper method to access labels under lock.
+func (s *memSeries) labels() labels.Labels {
+	s.Lock()
+	defer s.Unlock()
+	return s.lset
+}
diff --git a/tsdb/head_other.go b/tsdb/head_other.go
new file mode 100644
index 000000000..9306913d8
--- /dev/null
+++ b/tsdb/head_other.go
@@ -0,0 +1,25 @@
+// Copyright 2024 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build !dedupelabels
+
+package tsdb
+
+import (
+	"github.com/prometheus/prometheus/model/labels"
+)
+
+// Helper method to access labels; trivial when not using dedupelabels.
+func (s *memSeries) labels() labels.Labels {
+	return s.lset
+}
diff --git a/tsdb/head_read.go b/tsdb/head_read.go
index b47e24f9e..9ba8785ad 100644
--- a/tsdb/head_read.go
+++ b/tsdb/head_read.go
@@ -142,7 +142,7 @@ func (h *headIndexReader) SortedPostings(p index.Postings) index.Postings {
 	}
 
 	slices.SortFunc(series, func(a, b *memSeries) int {
-		return labels.Compare(a.lset, b.lset)
+		return labels.Compare(a.labels(), b.labels())
 	})
 
 	// Convert back to list.
@@ -189,7 +189,7 @@ func (h *headIndexReader) Series(ref storage.SeriesRef, builder *labels.ScratchB
 		h.head.metrics.seriesNotFound.Inc()
 		return storage.ErrNotFound
 	}
-	builder.Assign(s.lset)
+	builder.Assign(s.labels())
 
 	if chks == nil {
 		return nil
@@ -259,7 +259,7 @@ func (h *headIndexReader) LabelValueFor(_ context.Context, id storage.SeriesRef,
 		return "", storage.ErrNotFound
 	}
 
-	value := memSeries.lset.Get(label)
+	value := memSeries.labels().Get(label)
 	if value == "" {
 		return "", storage.ErrNotFound
 	}
@@ -283,7 +283,7 @@ func (h *headIndexReader) LabelNamesFor(ctx context.Context, series index.Postin
 			// when series was garbage collected after the caller got the series IDs.
 			continue
 		}
-		memSeries.lset.Range(func(lbl labels.Label) {
+		memSeries.labels().Range(func(lbl labels.Label) {
 			namesMap[lbl.Name] = struct{}{}
 		})
 	}
diff --git a/tsdb/head_wal.go b/tsdb/head_wal.go
index 41f7dd46b..787cb7c26 100644
--- a/tsdb/head_wal.go
+++ b/tsdb/head_wal.go
@@ -126,7 +126,7 @@ func (h *Head) loadWAL(r *wlog.Reader, syms *labels.SymbolTable, multiRef map[ch
 			}
 			// At the moment the only possible error here is out of order exemplars, which we shouldn't see when
 			// replaying the WAL, so lets just log the error if it's not that type.
-			err = h.exemplars.AddExemplar(ms.lset, exemplar.Exemplar{Ts: e.T, Value: e.V, Labels: e.Labels})
+			err = h.exemplars.AddExemplar(ms.labels(), exemplar.Exemplar{Ts: e.T, Value: e.V, Labels: e.Labels})
 			if err != nil && errors.Is(err, storage.ErrOutOfOrderExemplar) {
 				level.Warn(h.logger).Log("msg", "Unexpected error when replaying WAL on exemplar record", "err", err)
 			}
@@ -448,7 +448,7 @@ func (h *Head) resetSeriesWithMMappedChunks(mSeries *memSeries, mmc, oooMmc []*m
 			) {
 				level.Debug(h.logger).Log(
 					"msg", "M-mapped chunks overlap on a duplicate series record",
-					"series", mSeries.lset.String(),
+					"series", mSeries.labels().String(),
 					"oldref", mSeries.ref,
 					"oldmint", mSeries.mmappedChunks[0].minTime,
 					"oldmaxt", mSeries.mmappedChunks[len(mSeries.mmappedChunks)-1].maxTime,
@@ -932,7 +932,7 @@ func (s *memSeries) encodeToSnapshotRecord(b []byte) []byte {
 
 	buf.PutByte(chunkSnapshotRecordTypeSeries)
 	buf.PutBE64(uint64(s.ref))
-	record.EncodeLabels(&buf, s.lset)
+	record.EncodeLabels(&buf, s.labels())
 	buf.PutBE64int64(0) // Backwards-compatibility; was chunkRange but now unused.
 
 	s.Lock()
@@ -1485,7 +1485,7 @@ Outer:
 					continue
 				}
 
-				if err := h.exemplars.AddExemplar(ms.lset, exemplar.Exemplar{
+				if err := h.exemplars.AddExemplar(ms.labels(), exemplar.Exemplar{
 					Labels: e.Labels,
 					Value:  e.V,
 					Ts:     e.T,
diff --git a/tsdb/ooo_head_read.go b/tsdb/ooo_head_read.go
index 47972c3cc..4e8329c99 100644
--- a/tsdb/ooo_head_read.go
+++ b/tsdb/ooo_head_read.go
@@ -78,7 +78,7 @@ func (oh *OOOHeadIndexReader) series(ref storage.SeriesRef, builder *labels.Scra
 		oh.head.metrics.seriesNotFound.Inc()
 		return storage.ErrNotFound
 	}
-	builder.Assign(s.lset)
+	builder.Assign(s.labels())
 
 	if chks == nil {
 		return nil