The Shimna River (Irish: Simhné, meaning river of bulrushes) is a river in County Down, Northern Ireland. It rises on the slopes of Ott Mountain, in the Mourne Mountains, and enters the Irish Sea at Newcastle, on Dundrum Bay. It is acidic and nutrient-poor, as a result of which its most common flora are mosses and liverworts, including the rare Portuguese feather-moss and Holt's mouse-tail moss. Its principal fish are salmon and sea trout, and it is managed by the Shimna Angling Club.