| Mk | Entry | Card | Episode |
| Xanthras III. | 003.3zMM.[F | Menage a Troi. [TNG] | |
| Xanthras III. | 104.1A | Menage a Troi. [TNG] | |
| * | Xarantine. | 104.4A | Sleeping Dogs. [ENT] |
| Xelo. | 104.1B | Haven. [TNG] | |
| Xendi Sabu star system. | 104.1C | Battle, The. [TNG] | |
| Xendi Starbase 9. | 104.1D | Battle, The. [TNG] | |
| xenon-based lifeform. | 104.1F | Hope and Fear. [VGR] | |
| xenopolycythemia. | 007.21D.[D | For the World Is Hollow and I Have Touched the Sky. [TOS] | |
| xenopolycythemia. | 104.1G | For the World Is Hollow and I Have Touched the Sky. [TOS] | |
| Xepolites. | 104.1H | Maquis, The, Part II. [DS9] | |
| Xepolites. (Freighters) | 040APP1[AS | Maquis, The, Part II. [DS9] | |
| Xerses VII. | 003.3zMM.[G | When the Bough Breaks. [TNG] | |
| Xerxes VII. | 104.1I | When the Bough Breaks. [TNG] | |
| Xhosa." | 040.004.[F | Way of the Warrior, The, Parts I & II. [DS9] | |
| Xhosa." | 040.012. | Way of the Warrior, The, Parts I & II. [DS9] | |
| Xhosa." | 104.1J | Way of the Warrior, The, Parts I & II. [DS9] | |
| Xhosa." (Good ship, reliable crew) | 044.16[A | Way of the Warrior, The, Parts I & II. [DS9] | |
| Xhosa." (Petarian) (Freighters) | 040APP1[AT | Way of the Warrior, The, Parts I & II. [DS9] | |
| xupta tree. | 104.1K | Paradise. [DS9] | |
| * | Xyrillians. | 104.4B | Unexpected. [ENT] |
| * | Xyrillians. (reproduction) | 206.005-b | Unexpected. [ENT] |
| * | Xyrillians. (unconventional non-Federation ships) | 040APP3.k | Unexpected. [ENT] |