In Greek and European bestiaries and legends, a basilisk (from the Greek basileus, a king) is a legendary reptile reputed to be king of serpents and said to have the power of causing death by a single glance. According to the Naturalis Historia of Pliny the Elder, the basilisk is a small snake that is so poisonous that it leaves a wide trail of deadly venom in its wake, and its gaze is likewise lethal.

It is called "king" because it is reputed to have on its head a mitre-shaped crest. Stories of the basilisk place it in the same general family as the cockatrice. The basilisk is fabulously alleged to be hatched by a serpent or reptile from a cock's egg (in some legends specifically an egg laid by a seven-year-old cock during a full moon, or under various other astrological portents). In Medieval Europe, the description of the creature began taking on features from cockerels. Geoffrey Chaucer featured a basilicok (as he called it) in his Canterbury Tales.

Stories gradually added to the basilisk's deadly capabilities, such as describing it as a larger beast, capable of breathing fire and killing with the sound of its voice. Some writers even claimed that it could kill not only by touch, but also by touching something that is touching it, like a sword. The Basilisk is also the guardian creature of the Swiss city Basel.

Basilisks have been re-used in fantasy fiction for books, movies, and games. It is common to find basilisks in the bestiaries of role-playing games such as Dungeons & Dragons and Final Fantasy. A large, snake-like basilisk was also featured in Harry Potter and the Chamber of Secrets.