| Name | CR | Type | Environment |
| Ubashki, Ubashki Lynx | 6 | undead | temperate or warm deserts |
| Ubashki, Ubashki Swarm | 2 | undead (swarm) | warm deserts |
| Ugash-Iram | 15 | magical beast (aquatic)
Unique gutaki (variant devilfish) | — |
| Ugothokra | 7 | aberration | any |
| Umasi | 4 | monstrous humanoid | tropical jungle |
| Umbral Shepherd | 5 | outsider (incorporeal) | any (Plane of Shadow) |
| Umdhlebi | 12 | plant | tropical forests and jungles |
| Undead Pirate, Draugr Captain | 4 | undead (water) | any oceans or coastlines |
| Undead Pirate, Draugr Pirate | 3 | undead (water) | any oceans or coastlines |
| Undead Pirate, Ghost Captain | 7 | undead (augmented humanoid, incorporeal) | any oceans or coastlines |
| Undigested | 1/4 | undead | any |
| Undigested, Undigested Swarm | 5 | undead (swarm) | any |
| Undine | 1/2 | outsider (native) | any land |
| Unfettered Phantom | 4 | outsider (phantom) | any (Ethereal Plane or Material Plane) |
| Unicorn | 3 | magical beast | temperate forests |
| Unknown | 11 | fey (augmented humanoid) | any urban |
| Unrisen | 5 | undead | any |
| Uraeus | 5 | magical beast | warm rivers |
| Urannag | 8 | construct (extraplanar) | any swamps (the Abyss) |
| Urdefhan | 3 | outsider (native) | any land (Abaddon) |
| Urdefhan, Half-Fiend Scion | 15 | outsider (native) | any land (Abaddon) |
| Urdefhan, Orvian Necromancer | 12 | outsider (native) | any land (Abaddon) |
| Urdefhan, Sightless Sea Sailor | 4 | outsider (native) | any land (Abaddon) |
| Urdefhan, Skaveling Rider | 10 | outsider (native) | any land (Abaddon) |
| Urhag | 3 | aberration | cold mountains |
| Ursikka | 10 | magical beast | any cold land |
| Urtleytlar, The Tempest Queen | 20 | aberration (aquatic) | — |