SM U-121 was a Type UE II long-range minelaying U-boat of the Imperial German Navy intended for service in the Mediterranean. The Austro-Hungarian Navy allocated her the number SM U-84. She was built at Hamburg, Germany, by Aktiengesellschaft Vulcan and launched on 20 September 1918. Completed after the Armistice she was never commissioned in the Imperial German Navy but handed over to France on 9 March 1919. She was sunk as target off Cherbourg on 1 July 1921.