- u ... Ualan Island
- Uale, Frankie ... Uca pugnax
- Ucayali River ... Udjo
- UDMA ... Ughoton
- Uglich ... ʿUkāẓ
- ukelele ... Uleåborg
- ulema ... Ulster Unionist Party
- Ulster Volunteer Force ... ultraviolet-visible spectrophotometry
- Ultravox ... umbra
- Umbra limi ... UN
- Un ... Uncle Tom’s Cabin
- Uncle Tom’s Cabin; or, Life Among the Lowly ... Underflow Tunnel and Reservoir Plan
- underfriction wheel ... undifferentiated schizophrenia
- Undine ... unguligrade posture
- Ungureanu, Mihai Răzvan ... unilineal descent
- UNIMA ... Union Pacific Railroad Company
- Union Pacific System ... Unit, the
- UNITA ... United Independent Broadcasters, Inc.
- United International Bureau for the Protection of Intellectual Property ... United Nations/African Union Mission in Darfur
- United Negro College Fund ... United States expedition to Japan
- United States Federal Witness Protection Program ... United States presidential election of 1992
- United States presidential election of 1996 ... unity
- unity ... Universalism
- Universalist Church of America ... unlisted market
- Unlocking the power of the Chaikin money flow indicator ... UNTAG
- Untamed ... Up the Down Staircase
- Up the Sandbox ... upper delta plain
- Upper Devonian Series ... UPU
- Upupa epops ... Uranus
- Uranus, Operation ... ureilite
- uremia ... Urnfield culture
- URNG ... Urteil, Das
- Urtica ... Use of AI on Polish Radio Station Causes Outrage
- use of force ... USS Liberty incident
- USS Minnesota ... utai
- Utakata/Sankuchuari ... utricular macula
- Utricularia ... Uyuni
- Uyuni Salt Flat ... Uzziah