Macewen's sign is a sign used to help to diagnose hydrocephalus (accumulation of excess cerebrospinal fluid) and brain abscesses. Tapping (percussion ) the skull near the junction of the frontal, temporal and parietal bones will produce a stronger resonant sound when either hydrocephalus or a brain abscess are present.

The sign was discovered and described by Sir William Macewen (1848-1924), a surgeon and professor of the University of Glasgow, Scotland who also described Macewen's operation for inguinal hernia.