  We present an exact plane wave solution of the most general shift-symmetric
Horndeski (generalized Galileon) theory. The solution consists of the scalar
part, and the gravitational part with two polarization modes. The former is due
to the presence of the non-trivial Galileon scalar field, and it is
parametrized by an arbitrary function of the light-cone coordinate. For a
trivial scalar field configuration the solution is equivalent to the plane
gravitational wave in General Relativity. When the metric is Minkowski, we
reproduce known results for the plane waves of k-essence and a soliton-like
solutions of a non-covariant Galileon model in a flat space-time.
