Kaecilius (/kaɪˈsɪliəs/) is a supervillain appearing in American comic books published by Marvel Comics, primarily as an enemy of Doctor Strange. The character first appeared in Strange Tales #130 (March 1965) and was created by Stan Lee and Steve Ditko.

Kaecilius made his live-action debut in the Marvel Cinematic Universe film Doctor Strange, portrayed by Mads Mikkelsen.

Kaecilius's first appeared was in Strange Tales #130 (March 1965). He was created by Stan Lee and Steve Ditko.

Later appearances include:

Kaecilius is a disciple of Baron Mordo, who delivers messages and even fights Doctor Strange himself if necessary. One of his first missions is to kidnap the Ancient One, a task he fails due to Doctor Strange's interference. At one point, Mordo possesses Kaecilius to test the powers of Dormammu.

Kaecilius steals Strange's Cloak of Levitation before he erases Kaecilius's knowledge of magic.

Kaecilius and his cohorts, Adria and Demonicus, pose as members of the Independent Video media network to interview Strange and gain access to the Sanctum Sanctorum. Claiming to be the cameraman Joe Crocker, Kaecilius is exposed and banished to the Purple Dimension.

Many years later, he is freed and battles Strange and Jane Foster.