| Name | Source | Type | | Kadi | Star Trek |
| Kafers | 2300 AD | Humanoid |
| Kaleds | Doctor Who | Humanoid |
| Kalish | Farscape | Humanoid |
| Kaalium | Moontrap | Cyborg |
| Kal-Kriken | Utopia |
| Kalliran | C. J. Cherryh's Alliance-Union universe |
| Kambuchka | Ascendancy |
| Kanamit | To Serve Man Twilight Zone and Short Story | Humanoid |
| Kang and Kodos | The Simpsons | Squid-like. Green, single-eyed squids intent on conquering Earth |
| Karemma | Star Trek | Humanoid |
| Kariak | Legions of Iron |
| Kat | Kid vs. Kat | cybernetic, cat-like alien |
| Kazon | Star Trek | Humanoid |
| Kdatlyno | Larry Niven's Known Space |
| Kerons | Sgt. Frog |
| Kess'Rith | Renegade Legion | reptilian rhino-like centauroid |
| Key-Guardians | Utopia |
| King Ghidorah | Godzilla films | Three-headed dragon |
| Kineceleran | Ben 10 | Blue-skinned aliens resembling armored velociraptors with wheels on their feet, Kinecelarans hail from the stormy planet Kinet, and as a result of constant electrical storms, they have extraordinary speed, able to accelerate from 0 to 500 miles per hour in a second. They can use their speed to generate twisters, and also have prehensile tails and sharp claws. |
| Kisshu | Tokyo Mew Mew |
| Kharaa | Natural Selection |
| Kherubim | Wildstorm |
| Khunds | DC Comics | Humanoid |
| Khurtarnan | Farscape | Humanoid |
| Kif | C. J. Cherryh's Chanur novels |
| Kig-yar | Halo |
| Kilaaks | Destroy All Monsters |
| Kilrathi | Wing Commander games |
| Kizanti | Battlelords of the 23rd Century | Humanoid |
| Kimera | Final Conflict |
| King Cold | Dragon Ball Z | Humanoid; Reptilian |
| Kivar | Roswell |
| Klaestron | Star Trek |
| Klackons | Master of Orion |
| Klaxuns | 2300 AD |
| Kleer Skeleton | The First Encounter |
| Klingons | Star Trek | Humanoid |
| Knnn | C. J. Cherryh's Chanur novels |
| Koozbanians | The Muppet Show |
| Korath | Dark Avatar |
| Korbinites | Marvel Comics |
| Korvax | No Man's Sky | Humanoid; Machine |
| Korx | Galactic Civilizations: Altarian Prophecy |
| K-PAXians | K-PAX |
| Kra'hen | Alliances |
| Krakeds | Battlelords of the 23rd Century |
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| Kree | Marvel Comics | Humanoid |
| Kreel | Star Trek |
| Kreely | Schlock Mercenary |
| Kreetassan | Star Trek |
| Krell | Forbidden Planet | |
| Krellepem | Harry Turtledove's Worldwar series | An aquatic animal native to Home. Their uncanny physiology resembled Earth's extinct trilobites and they were implied to be less evolved. Krellepem are treated as a delicacy by the Race in which special tools are required to break their shells, especially since the Race could not even suck the meat out of the shells as humans could do. |
| Krenim | Star Trek |
| Kriken | Metroid Prime Hunters |
| Krishnans | L. Sprague de Camp's Viagens Interplanetarias Series | Green skinned humanoids with antennae. |
| Krith | Richard C. Meredith's Timeliner Trilogy |
| Krogan | Mass Effect |
| Krolp | Harry Turtledove's Vilcacamba | Centauroids larger than horses with tiger-like stripes, heads similar in shape to a jack-o'-lantern with jagged jaws, and have a strong odor that similar in smell to Limburger cheese. |
| Kromaggs | Sliders |
| Kronn | Area 51 and its sequel | One-eyed insectoid. Able to mutate humans, turning them into zombies, cling on walls, and fires some sort of energy. |
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| Kroot | Warhammer 40,000 |
| Krynn | Dark Avatar |
| Krynn | Buried in Time |
| Krynoids | Doctor Who |
| Kryptonians | Superman | Humanoid |
| Kryten | Red Dwarf | Series 4000 Mechanoid |
| Kssthrata | Schlock Mercenary |
| K'tang | Star Control 3 |
| Ktarian | Star Trek |
| Kulturan | The Sinusoidal Spaghetti |
| Kweltikwan | Lilo & Stitch | Characterized by a large, rotund figure, four yellow eyes and a dark blue-ish/purple skin tones save for pink on their chest/stomachs, large wide mouths, blunted fangs and small noses. |
| Kymellians | Marvel Comics | Humanoid body with horses head |
| Kymnar | 2448 RPG |
| Kyo | C. J. Cherryh's Foreigner series |
| Kyrie | Heroscape | Humans with wings |
| Kyulek | Off-World Resource Base |
| Kzinti | Larry Niven's Known Space series and The Animated Series | Felinoid - large bi-pedal tigers. Male: Kzintosh, Female: Kzinrett. Aggressive space-faring carnivores, with sub-sapient females |