Greenan Castle is a 16th-century tower house, possibly on the site of an ancient fort, around 2.5 miles south-west of Ayr in South Ayrshire, Scotland. The entrance lintel has the inscription, JK 1603 FMD, for John Kennedy of Baltersan and his third wife, Florence MacDowell, who held the lands, Greenan Mill, and salmon fishing rights on the River Doon at that time.
Beside the tower are traces of a walled courtyard and outbuildings - probably stables and a kitchen block as the small tower has no kitchen within its walls. In this courtyard on the morning of 12 May 1602, Sir Thomas Kennedy of Culzean and his servant, Lancelot Kennedy, mounted their horses to ride to Edinburgh, having spent the night before with Thomas's half-brother, John Kennedy of Baltersan. Just a few miles away in the woods of St Leonards (now a suburb of Ayr), they were ambushed by Thomas Kennedy of Drummurchie, Thomas Kennedy, brother to the Laird of Bargany, Walter Muir of Cloncaird, Thomas M'Alexander, Thomas Wallace, a boy called Gilbert Ramsay and a borderer, Williame Irrwing. Sir Thomas was murdered in retaliation for the death of the young Laird of Bargany in December, 1601 at the Battle of Brockloch, near Maybole. Years later, the Muirs of Auchindrain (father and son) were executed for their "art and part" in this murder. The story inspired Sir Walter Scott to write a short play, "An Ayrshire Tragedy".