The most common symbol of Antifa, a loose collection of far-left activist groups, is a black and red double flag. Some European anti-fascist groups have used a red triangle in the past, but they appear to have done so to refer to a symbol of violence used originally against leftist political activists. Mark Bray, in the The Antifascist Handbook, cites its use by a British anti-fascist group in the 1980s.
The red triangle symbol was used by Nazis to mark political prisoners in concentration camps during World War II. It gained attention recently, when used in a Trump campaign ad focused on Antifa.