Greenan Castle is a 16th-century ruined tower house, around 2+1⁄2 miles (4 kilometres) southwest of Ayr in South Ayrshire, Scotland.

[1] The lands of Greenan were forfeited by John, Earl of Ross and Lord of the Isles in 1476 for treason against James III.

Four concentric curved ditches, each around 3 metres (10 ft) wide, cut off the headland with the exception of a single causeway to provide access.

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.

Around this time the residential use of the castle seems to have stopped; today the structure is a ruin 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.