The Zenneck wave, Zenneck surface wave or Sommerfeld-Zenneck surface wave is a longitudinal, inhomogeneous or non-uniform electromagnetic plane wave incident at the complex Brewster's angle onto a planar or spherical boundary interface between two homogeneous media having different dielectric constants.

The Zenneck wave propagates parallel to the interface and decays exponentially vertical to it, a property known as evanescence.