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.