Solar cells used in space are subject to radiation degradation by energetic particles (electrons, protons, cosmic rays). Our work studies the effect of irradiation by 1 MeV electrons on a GaAs-based solar cell of the (N+)GaAs/(P)GaAs type, using a simulation code developed in MATLAB. GaAs is chosen due to its direct band gap (1.42 eV) and its radiation resistance. 1 MeV electrons are considered as the most representative to evaluate cell degradation. The presented model takes into account the diffusion and recombination current in the dark, the photocurrent under illumination and the introduction of deep levels in the GaAs band gap (E1–E5 for N-type GaAs and H0–H3 for P-type GaAs) created by irradiation defects. The effect of each level on the cell output parameters (short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF) and maximum power (Pm)) is studied. Results show that irradiation strongly decreases Voc, FF and Pm, while Jsc remains almost unchanged because it depends mainly on the absorption coefficient of the material. Energy levels E2 and especially E5 have a major effect on the degradation. The deep level E5, identified as the dominant recombination center, is responsible for a power loss of up to 70% for a fluence of 1018 electrons/m2. The degradation of GaAs-based cells under 1 MeV electrons is therefore dominated by the carrier recombination phenomenon via deep levels (in particular E5). Mitigation solutions are proposed such as reducing the junction thickness, doping optimization, and surface passivation processes.