diff --git a/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp b/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
index f00b99075b022..2da41348244d7 100644
--- a/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
+++ b/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
@@ -3025,7 +3025,7 @@ bool WaitcntBrackets::mergeAsyncMarks(ArrayRef<MergeInfo> MergeInfos,
   bool StrictDom = false;
 
   LLVM_DEBUG(dbgs() << "Merging async marks ...");
-  // Early exit: both empty
+  // Early exit: nothing to merge when both sides are empty.
   if (AsyncMarks.empty() && OtherMarks.empty()) {
     LLVM_DEBUG(dbgs() << " nothing to merge\n");
     return false;
@@ -3067,6 +3067,11 @@ bool WaitcntBrackets::mergeAsyncMarks(ArrayRef<MergeInfo> MergeInfos,
   unsigned OtherSize = OtherMarks.size();
   unsigned OurSize = AsyncMarks.size();
   unsigned MergeCount = std::min(OtherSize, OurSize);
+  // OtherMarks is empty -> OtherSize == 0 -> MergeCount == 0.
+  // Our existing marks are the conservative result; return early to avoid
+  // passing MergeCount == 0 to seq_inclusive which asserts Begin <= End.
+  if (MergeCount == 0)
+    return StrictDom;
   for (auto Idx : seq_inclusive<unsigned>(1, MergeCount)) {
     for (auto T : inst_counter_types(Context->MaxCounter)) {
       StrictDom |= mergeScore(MergeInfos[T], AsyncMarks[OurSize - Idx][T],
diff --git a/llvm/test/CodeGen/AMDGPU/asyncmark-merge-empty-other.mir b/llvm/test/CodeGen/AMDGPU/asyncmark-merge-empty-other.mir
new file mode 100644
index 0000000000000..cf3a46d39fcb2
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/asyncmark-merge-empty-other.mir
@@ -0,0 +1,139 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 6
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx1250 -run-pass=si-insert-waitcnts -o - %s | FileCheck %s
+
+# Regression test for mergeAsyncMarks() asserting when OtherMarks is empty.
+#
+# At a CFG join point where one predecessor has an ASYNCMARK (non-empty
+# AsyncMarks) and the other does not (empty OtherMarks), MergeCount becomes
+# min(0, N) = 0. Before the fix, seq_inclusive<unsigned>(1, 0) asserted
+# Begin <= End. After the fix the function returns early when either side
+# is empty.
+#
+# GLOBAL_LOAD_ASYNC_TO_LDS_B32 is a GFX1250 async LDS DMA instruction tracked
+# via ASYNC_CNT.  isAsync() returns true for it, so the score is recorded into
+# AsyncScore before ASYNCMARK pushes it onto AsyncMarks.
+#
+# The join block contains WAIT_ASYNCMARK 0 to consume the pending mark.
+# Before the fix, mergeAsyncMarks() asserted before reaching the wait.
+# After the fix the pass completes without asserting.
+#
+# Two patterns are tested:
+#   asyncmark_in_then - ASYNCMARK in the then-successor, else-successor is sync
+#   asyncmark_in_else - ASYNCMARK in the else-successor, then-successor is sync
+
+---
+# Pattern 1: ASYNCMARK in then-successor, else-successor is sync.
+name:            asyncmark_in_then
+tracksRegLiveness: true
+machineFunctionInfo:
+  occupancy:       8
+body:             |
+  ; CHECK-LABEL: name: asyncmark_in_then
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.2(0x40000000), %bb.1(0x40000000)
+  ; CHECK-NEXT:   liveins: $sgpr0, $sgpr1, $vgpr0_vgpr1, $vgpr2
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   S_WAIT_LOADCNT_DSCNT 0
+  ; CHECK-NEXT:   S_WAIT_KMCNT 0
+  ; CHECK-NEXT:   S_CMP_LG_U32 $sgpr0, $sgpr1, implicit-def $scc
+  ; CHECK-NEXT:   S_CBRANCH_SCC1 %bb.2, implicit killed $scc
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
+  ; CHECK-NEXT:   liveins: $vgpr0_vgpr1, $vgpr2
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   GLOBAL_LOAD_ASYNC_TO_LDS_B32 $vgpr2, $vgpr0_vgpr1, 0, 0, implicit-def $asynccnt, implicit $exec, implicit $asynccnt :: (load (s32), addrspace 1), (store (s32), addrspace 3)
+  ; CHECK-NEXT:   ASYNCMARK
+  ; CHECK-NEXT:   S_BRANCH %bb.3
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   S_BRANCH %bb.3
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3:
+  ; CHECK-NEXT:   WAIT_ASYNCMARK 0
+  ; CHECK-NEXT:   S_WAIT_ASYNCCNT 0, implicit-def $asynccnt, implicit $asynccnt
+  ; CHECK-NEXT:   S_ENDPGM 0
+  bb.0:
+    liveins: $sgpr0, $sgpr1, $vgpr0_vgpr1, $vgpr2
+
+    S_CMP_LG_U32 $sgpr0, $sgpr1, implicit-def $scc
+    S_CBRANCH_SCC1 %bb.2, implicit killed $scc
+
+  ; then branch — issues async LDS DMA + ASYNCMARK
+  bb.1:
+    liveins: $vgpr0_vgpr1, $vgpr2
+
+    GLOBAL_LOAD_ASYNC_TO_LDS_B32 $vgpr2, $vgpr0_vgpr1, 0, 0, implicit-def $asynccnt, implicit $exec, implicit $asynccnt :: (load (s32), addrspace 1), (store (s32), addrspace 3)
+    ASYNCMARK
+    S_BRANCH %bb.3
+
+  ; else branch — sync path, no async operations; OtherMarks is empty at join
+  bb.2:
+    S_BRANCH %bb.3
+
+  ; join — mergeAsyncMarks sees non-empty AsyncMarks (then) and empty OtherMarks (else).
+  ; Before fix: assertion. After fix: returns early, no spurious wait inserted.
+  bb.3:
+    WAIT_ASYNCMARK 0
+    S_ENDPGM 0
+...
+---
+# Pattern 2: ASYNCMARK in else-successor, then-successor is sync.
+# Mirror of asyncmark_in_then — exercises the opposite predecessor ordering.
+name:            asyncmark_in_else
+tracksRegLiveness: true
+machineFunctionInfo:
+  occupancy:       8
+body:             |
+  ; CHECK-LABEL: name: asyncmark_in_else
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.2(0x40000000), %bb.1(0x40000000)
+  ; CHECK-NEXT:   liveins: $sgpr0, $sgpr1, $vgpr0_vgpr1, $vgpr2
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   S_WAIT_LOADCNT_DSCNT 0
+  ; CHECK-NEXT:   S_WAIT_KMCNT 0
+  ; CHECK-NEXT:   S_CMP_LG_U32 $sgpr0, $sgpr1, implicit-def $scc
+  ; CHECK-NEXT:   S_CBRANCH_SCC1 %bb.2, implicit killed $scc
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   S_BRANCH %bb.3
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
+  ; CHECK-NEXT:   liveins: $vgpr0_vgpr1, $vgpr2
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   GLOBAL_LOAD_ASYNC_TO_LDS_B32 $vgpr2, $vgpr0_vgpr1, 0, 0, implicit-def $asynccnt, implicit $exec, implicit $asynccnt :: (load (s32), addrspace 1), (store (s32), addrspace 3)
+  ; CHECK-NEXT:   ASYNCMARK
+  ; CHECK-NEXT:   S_BRANCH %bb.3
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3:
+  ; CHECK-NEXT:   WAIT_ASYNCMARK 0
+  ; CHECK-NEXT:   S_WAIT_ASYNCCNT 0, implicit-def $asynccnt, implicit $asynccnt
+  ; CHECK-NEXT:   S_ENDPGM 0
+  bb.0:
+    liveins: $sgpr0, $sgpr1, $vgpr0_vgpr1, $vgpr2
+
+    S_CMP_LG_U32 $sgpr0, $sgpr1, implicit-def $scc
+    S_CBRANCH_SCC1 %bb.2, implicit killed $scc
+
+  ; then branch — sync path, no async operations
+  bb.1:
+    S_BRANCH %bb.3
+
+  ; else branch — issues async LDS DMA + ASYNCMARK
+  bb.2:
+    liveins: $vgpr0_vgpr1, $vgpr2
+
+    GLOBAL_LOAD_ASYNC_TO_LDS_B32 $vgpr2, $vgpr0_vgpr1, 0, 0, implicit-def $asynccnt, implicit $exec, implicit $asynccnt :: (load (s32), addrspace 1), (store (s32), addrspace 3)
+    ASYNCMARK
+    S_BRANCH %bb.3
+
+  ; join block
+  bb.3:
+    WAIT_ASYNCMARK 0
+    S_ENDPGM 0
+...