List of fictional extraterrestrial species and races: X
| Name | Source | Type |
| X-ists | Church of the SubGenius | |
| X-Nauts | Paper Mario: The Thousand-Year Door | |
| X Parasites | Metroid Fusion | |
| Xandarians | Marvel Comics | Humanoid |
| Xanthuans | DC Comics' Legion of Super-Heroes | |
| Xarians | Battlelords of the 23rd Century | |
| Xarquids | X-COM: Terror from the Deep | |
| Xchagger | Star Control | |
| Xeelee | Xeelee Sequence novels | an enigmatic and supremely powerful Kardashev Type V alien civilization |
| Xel'Naga | StarCraft | An ancient race that created the Zerg and Protoss. |
| Xenexian | Star Trek | |
| Xenomorphs | Alien movies | Parasitic |
| Xenu | Scientology | |
| Xiangs | 2300 AD | |
| Xiao Fengqing | Legend of Heavenly Tear: Phoenix Warriors | Humanoid |
| Xilians | Invasion of Astro-Monster and Godzilla: Final Wars | Humanoid |
| Xindi | Star Trek | Humanoid |
| Xorda | Sonic the Hedgehog comics | |
| Xxcha | Twilight Imperium | Peaceful arboreal testudinoids |
| Xyrillian | Star Trek |