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.