Colonel Alexander Ranaldson MacDonell of Glengarry, sometimes called by the Gaelic version of his name, Alastair or Alasdair, was clan chief of Clan MacDonell of Glengarry. As was customary for a laird, MacDonell was often called Glengarry after his principal estate.

Clan MacDonell of Glengarry, also known as Clan Ranald of Knoydart & Glengarry is a Scottish clan and is a branch of the larger Clan Donald. The clan takes its name from River Garry where the river Garry runs eastwards through Loch Garry to join the Great Glen about 16 miles (25 km) north of Fort William, Highland. The progenitor of the MacDonells of Glengarry is Reginald, 4th great-grandson of the warrior Somerled. The clan chief is traditionally designated as the "Son of Alexander's son".