U-109 may refer to one of the following German submarines:
, a Type U 93 submarine launched in 1917 that served in the First World War and sank in 1918 During the First World War, Germany also had this submarine with a similar name: , a Type UB III submarine launched in 1917 and sank in 1918 , a Type IXB submarine that served in the Second World War and sank in 1943