This paper presents a novel concept of a spaceborne Digital Beamforming (DBF) Synthetic Aperture Radar (SAR) based on a multi-beam reflectarray antenna. The proposed solution offers a number of advantages compared to more conventional DBF SAR systems using planar arrays or reflector antennas. The paper focuses on the spaceborne DBF SAR and discusses its architecture, system parameters and an imaging scenario. A reflectarray antenna design to be used in a combination with the considered imaging platform is presented in the paper. The reflectarray model is used to estimate DBF SAR performance characteristics discussed in the final part of the paper.