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