Widder (HSK 3) was an auxiliary cruiser (Hilfskreuzer) of the German Navy that was used as a merchant raider in the Second World War. Her Kriegsmarine designation was Schiff 21, to the Royal Navy she was Raider D. The name Widder (Ram) represents the constellation Aries in the German language. The name was given after the horoscope sign of the ship's master, Kapitän zur See Helmuth von Ruckteschell.