| U.S. Civil Rights Movement | U.S. Socialist Labour Party | 
| U.S. Socialist Party | Uceda, Luis de la Puente (1926-1965) | 
| Udyawar, S.P. | UHK (Union der Hand- und Kopfarbeiter) | 
| Ulbricht, Walter (1893-1973) | Umov, Nikolai Alekseevich (1846-1915) | 
| Underconsumption | Understanding | 
| Unfried, Emil (1892-1949) | Ungern von Sternberg, Roman (1886-1921) | 
| Union | Union des Jeunesses Comunistes (marxiste-léniniste) | 
| Union of Islamic Revolutionary Societies | Union of Oppositional Trade Unions | 
| Union of Soviet Socialist Republics (State) | Unione Syndicale Italiana | 
| Unionists | Unit of Analysis (Psychology) | 
| Unitary Communist Faction | United Front | 
| Unity of Opposites | Universal | 
| Universalism | Untermann, Gerhard Ernest Sr. (1864-1956) | 
| Urbahns, Hugo (1890-1946) | Uritsky, Moisei (1874-1918) | 
| Urquhart, John Leslie (1874-1933) | Use-Value | 
| USPD | Uspensky, Gleb (1840-1902) | 
| USSR (country) | USTICA | 
| Utilitarianism | Utility | 
| Utopia |