Abstract The design of a beam steerable Fabry‐Perot cavity antenna based on the use of a phase modulated partially reflective surface is reported. The latter partially reflective surface makes use of a 3D‐printed index‐modulated dielectric substrate so as to incorporate the phase modulation. The engineered substrate is combined with uniform inductive and capacitive grids in order to design the phase‐modulated metal‐dielectric‐metal metasurface acting as a partially reflecting surface in the cavity antenna. Thanks to the phase modulation, the radiated beam of the antenna is steered to an off‐normal direction.