SM U-1 or U-I was the lead boat of her class of submarine or U-boat built for and operated by the Austro-Hungarian Navy . U-1 was designed by American Simon Lake's Lake Torpedo Boat Company and built at the navy yard in Pola. She was one of two Lake-designed submarines purchased as part of a competitive evaluation of foreign submarine designs.

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

• U-1, the aircraft designation of the De Havilland Canada DHC-3 Otter light transport aircraft
• Multiple German U-Boats named U-1
• Oberursel U.I, an early German aircraft engine
• HDMS U-1, a Danish submarine

UI or Ui may refer to:
In science and technology:

• An alternate symbol for the international unit (IU)
• Unit interval (data transmission)
• Universal indicator, a type of pH indicator
• Unit Injector, a fuel injection technology for diesel engines
• Urinary incontinence
• User interaction
• User interface
• Ui (digraph), a digraph used in some writing systems
• Unemployment Insurance