diff --git a/CHANGELOG.md b/CHANGELOG.md
index 74f559818a..b699044e73 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 - [#5642](https://github.com/thanos-io/thanos/pull/5642) Receive: Log labels correctly in writer debug messages.
 - [#5655](https://github.com/thanos-io/thanos/pull/5655) Receive: Fix recreating already pruned tenants.
 - [#5702](https://github.com/thanos-io/thanos/pull/5702) Store: Upgrade minio-go/v7 to fix panic caused by leaked goroutines.
+- [#5736](https://github.com/thanos-io/thanos/pull/5736) Compact: Fix crash in GatherNoCompactionMarkFilter.NoCompactMarkedBlocks.
 
 ### Added
 * [#5654](https://github.com/thanos-io/thanos/pull/5654) Query: add `--grpc-compression` flag that controls the compression used in gRPC client. With the flag it is now possible to compress the traffic between Query and StoreAPI nodes - you get lower network usage in exchange for a bit higher CPU/RAM usage.
diff --git a/pkg/compact/compact.go b/pkg/compact/compact.go
index e17ea0f385..551e600b09 100644
--- a/pkg/compact/compact.go
+++ b/pkg/compact/compact.go
@@ -1361,6 +1361,7 @@ type GatherNoCompactionMarkFilter struct {
 	bkt                objstore.InstrumentedBucketReader
 	noCompactMarkedMap map[ulid.ULID]*metadata.NoCompactMark
 	concurrency        int
+	mtx                sync.Mutex
 }
 
 // NewGatherNoCompactionMarkFilter creates GatherNoCompactionMarkFilter.
@@ -1374,12 +1375,21 @@ func NewGatherNoCompactionMarkFilter(logger log.Logger, bkt objstore.Instrumente
 
 // NoCompactMarkedBlocks returns block ids that were marked for no compaction.
 func (f *GatherNoCompactionMarkFilter) NoCompactMarkedBlocks() map[ulid.ULID]*metadata.NoCompactMark {
-	return f.noCompactMarkedMap
+	f.mtx.Lock()
+	copiedNoCompactMarked := make(map[ulid.ULID]*metadata.NoCompactMark, len(f.noCompactMarkedMap))
+	for k, v := range f.noCompactMarkedMap {
+		copiedNoCompactMarked[k] = v
+	}
+	f.mtx.Unlock()
+
+	return copiedNoCompactMarked
 }
 
 // Filter passes all metas, while gathering no compact markers.
 func (f *GatherNoCompactionMarkFilter) Filter(ctx context.Context, metas map[ulid.ULID]*metadata.Meta, synced block.GaugeVec, modified block.GaugeVec) error {
+	f.mtx.Lock()
 	f.noCompactMarkedMap = make(map[ulid.ULID]*metadata.NoCompactMark)
+	f.mtx.Unlock()
 
 	// Make a copy of block IDs to check, in order to avoid concurrency issues
 	// between the scheduler and workers.
@@ -1389,9 +1399,8 @@ func (f *GatherNoCompactionMarkFilter) Filter(ctx context.Context, metas map[uli
 	}
 
 	var (
-		eg  errgroup.Group
-		ch  = make(chan ulid.ULID, f.concurrency)
-		mtx sync.Mutex
+		eg errgroup.Group
+		ch = make(chan ulid.ULID, f.concurrency)
 	)
 
 	for i := 0; i < f.concurrency; i++ {
@@ -1413,9 +1422,9 @@ func (f *GatherNoCompactionMarkFilter) Filter(ctx context.Context, metas map[uli
 					continue
 				}
 
-				mtx.Lock()
+				f.mtx.Lock()
 				f.noCompactMarkedMap[id] = m
-				mtx.Unlock()
+				f.mtx.Unlock()
 				synced.WithLabelValues(block.MarkedForNoCompactionMeta).Inc()
 			}