This work presents the design of a reflectarray fed by a planar leaky-wave antenna (LWA) for use in a 3U CubeSat. The LWA is based on substrate-integrated waveguide (SIW) technology and radiates in single-linear polarization. A full-wave simulation is carried out to obtain the near field radiated by the SIW LWA on a surface corresponding to the reflectarray, whose size is set to fit three sides of the CubeSat for stowage and deployment. A single-layered reflectarray unit cell consisting of three parallel dipoles has been chosen for simplicity and low profile. It provides a phase-shift range of more than 360° for layout design. In addition, the angle of incidence of the impinging near field is calculated by means of the Poynting vector. Simulation results at 24 GHz show the suitability of the design procedure for point-to-point communications, achieving an estimated gain of more than 30 dBi, taking into account substrate losses, and illumination and spillover efficiencies.