| Named By: | Richard Harlan in 1847 |
| Time Period: | Late Eocene |
| Location: | USA. Egypt, Wadi Al-Hitan. Pakistan |
| Size: | Average 18 meters long |
| Diet: | unavailable |
| Fossil(s): | Lots of known specimens |
| Classification: | | Chordata | Mammalia | Cetacea | Archaeoceti | Basilosauridae | |
| Also known as: | | Zeuglodon | |
Basilosaurus ("king lizard") is a genus of prehistoric cetacean that existed during the Late Eocene, 30 to 40 millions years ago (mya). The first fossil of B. cetoides was discovered in the United States and was initially believed to be some sort of reptile, hence the suffix -saurus, but it was later found to be a marine mammal. Richard Owen wished to rename the creature Zeuglodon ("yoked tooth"), but, per taxonomic rules, the creature's first name remained permanent. Fossils of B. isis have been found in Egypt and Jordan.
The species B. cetoides is the state fossil of Mississippi and Alabama.