U1

U1, U-I, U.I or U-1 may refer to:

Military

 * U-1, the USAF and US Military's designation for the De Havilland Canada DHC-3 Otter light transport aircraft
 * U-1, Soviet designation for Avro 504 trainer.
 * Multiple German U-Boats named U-1
 * HDMS U-1, a Danish submarine
 * SM U-1, an Austro-Hungarian submarine, lead ship of the U-1 class