Mars Cube One (MarCO) is the first CubeSat mission developed for operation at Mars. MarCO will provide a bent pipe telecom relay to Earth for NASA's InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission during its Entry, Descent, and Landing (EDL) sequence. The MarCO High Gain Antenna (HGA) is a key enabling technology for this mission and presents a tremendous technical challenge due to the limited mass and volume of a 6U CubeSat. This paper presents the design and performance of the circularly polarized X-Band deployable reflectarray developed for MarCO.