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U-1 may refer to one of the following German submarines:

• SM U-1 (Germany) (1906), Germany's first U-boat, now preserved in the Deutsches Museum in Munich; served in the First World War as a German training submarine for the Kaiserliche Marine; now a museum ship
  • During the First World War, Germany also had these submarines with similar names:
    • SM UB-1, a Type UB I submarine launched in 1915; transferred to Austria-Hungary on 12 July 1915 and renamed U-10; sunk on 9 July 1918
    • SM UC-1, a Type UC I submarine launched in 1915 and sunk in July 1917
• German submarine U-1 (1935), a Type IIA submarine that served in the Second World War and was sunk in April 1940
• German submarine U-1 (1961), a Type 201 submarine of the Bundesmarine, launched in 1961 and scrapped in 1967
• German submarine U-1 (1967), a Type 205 submarine of the Bundesmarine that was launched in 1967 and sold in 1991

U-1 or U-I may also refer to:

• SM U-1 (Austria-Hungary), lead boat of the U-1 class submarines for the Austro-Hungarian Navy