The authors present the design and implementation of a new type of micromachined terahertz planar highly directive antenna based on a periodic partially reflective surface. The periodic surface and antenna are fabricated using SU-8 photoresist micromachining technology. The proposed antenna is based on a Fabry–Perot type resonant cavity which is formed by superimposing the periodic surface over a single waveguide-fed slot in a metallic ground at a distance of about half wavelength. Simulations and measurements of the antenna performance are presented.