diff --git a/path/__init__.py b/path/__init__.py
index 33dd979..dadb4bd 100644
--- a/path/__init__.py
+++ b/path/__init__.py
@@ -1047,7 +1047,8 @@ def getatime(self) -> float:
         return self.module.getatime(self)
 
     def set_atime(self, value: float | datetime.datetime) -> None:
-        mtime_ns = self.stat().st_atime_ns
+        # Preserve mtime; st_atime_ns was a copy-paste bug (clobbered mtime).
+        mtime_ns = self.stat().st_mtime_ns
         self.utime(ns=(_make_timestamp_ns(value), mtime_ns))
 
     @property
diff --git a/tests/test_path.py b/tests/test_path.py
index b424bb6..b52b3bf 100644
--- a/tests/test_path.py
+++ b/tests/test_path.py
@@ -269,6 +269,15 @@ def test_utime(self, tmpdir):
         new_time = (time.time() - 600,) * 2
         assert Path(tmpfile).utime(new_time).stat().st_atime == new_time[0]
 
+    def test_set_atime_preserves_mtime(self, tmpdir):
+        tmpfile = Path(tmpdir) / 'file'
+        tmpfile.touch()
+        os.utime(tmpfile, (1000.0, 2000.0))
+        tmpfile.set_atime(3000.0)
+        st = tmpfile.stat()
+        assert st.st_atime == 3000.0
+        assert st.st_mtime == 2000.0
+
     def test_chmod_str(self, tmpdir):
         tmpfile = Path(tmpdir) / 'file'
         tmpfile.touch()