German submarine U-121 was a long-lived Type IIB U-boat built during World War II for service in the Kriegsmarine. U-121 spent the entire war as a training vessel and was scuttled at the end of the conflict. U-121 was one of two Type II U-boats built at Flender Werke in Lübeck. Like her sister boat (also built in Lübeck), she was originally constructed for export to China. The advent of World War II and the increased training needs of the U-bootwaffe led the German high command to assign U-120 and U-121 to the training command instead.