diff --git a/test/unittests/base/atomic-utils-unittest.cc b/test/unittests/base/atomic-utils-unittest.cc
index 06256764b6..794d4c4a50 100644
--- a/test/unittests/base/atomic-utils-unittest.cc
+++ b/test/unittests/base/atomic-utils-unittest.cc
@@ -207,5 +207,80 @@ TEST(AsAtomic8, CompareAndSwap_Concurrent) {
   }
 }
 
+TEST(AsAtomicWord, SetBits_Sequential) {
+  uintptr_t word = 0;
+  // Fill the word with a repeated 0xF0 pattern.
+  for (unsigned i = 0; i < sizeof(word); i++) {
+    word = (word << 8) | 0xF0;
+  }
+  // Check the pattern.
+  for (unsigned i = 0; i < sizeof(word); i++) {
+    EXPECT_EQ(0xF0u, (word >> (i * 8) & 0xFFu));
+  }
+  // Set the i-th byte value to i.
+  uintptr_t mask = 0xFF;
+  for (unsigned i = 0; i < sizeof(word); i++) {
+    uintptr_t byte = static_cast<uintptr_t>(i) << (i * 8);
+    AsAtomicWord::SetBits(&word, byte, mask);
+    mask <<= 8;
+  }
+  for (unsigned i = 0; i < sizeof(word); i++) {
+    EXPECT_EQ(i, (word >> (i * 8) & 0xFFu));
+  }
+}
+
+namespace {
+
+class BitSettingThread final : public Thread {
+ public:
+  BitSettingThread()
+      : Thread(Options("BitSettingThread")),
+        word_addr_(nullptr),
+        bit_index_(0) {}
+
+  void Initialize(uintptr_t* word_addr, int bit_index) {
+    word_addr_ = word_addr;
+    bit_index_ = bit_index;
+  }
+
+  void Run() override {
+    uintptr_t bit = 1;
+    bit = bit << bit_index_;
+    AsAtomicWord::SetBits(word_addr_, bit, bit);
+  }
+
+ private:
+  uintptr_t* word_addr_;
+  int bit_index_;
+};
+
+}  // namespace.
+
+TEST(AsAtomicWord, SetBits_Concurrent) {
+  const int kBitCount = sizeof(uintptr_t) * 8;
+  const int kThreadCount = kBitCount / 2;
+  BitSettingThread threads[kThreadCount];
+
+  uintptr_t word;
+  AsAtomicWord::Relaxed_Store(&word, 0);
+  for (int i = 0; i < kThreadCount; i++) {
+    // Thread i sets bit number i * 2.
+    threads[i].Initialize(&word, i * 2);
+  }
+  for (int i = 0; i < kThreadCount; i++) {
+    threads[i].Start();
+  }
+  for (int i = 0; i < kThreadCount; i++) {
+    threads[i].Join();
+  }
+  uintptr_t actual_word = AsAtomicWord::Relaxed_Load(&word);
+  for (int i = 0; i < kBitCount; i++) {
+    // Every second bit must be set.
+    uintptr_t expected = (i % 2 == 0);
+    EXPECT_EQ(expected, actual_word & 1u);
+    actual_word >>= 1;
+  }
+}
+
 }  // namespace base
 }  // namespace v8