Flatt Linn is a waterfall on the Rotten Calder Water, a river in South Lanarkshire, Scotland, east of East Kilbride. It is one of several notable waterfalls along this river, which is celebrated for its romantic gorge scenery, lush crags, and wooded valley.

Flatt Linn is also referred to as Crutherland Linn in some sources.

The Rotten Calder's many linns — a Scots term for a waterfall or deep pool — have drawn visitors for centuries, and Flatt Linn sits among a sequence that includes Torrance Linn, Black Linn, Trough Linn, and Calderwood Linn, among others.