In this work, the effects of rare earth ion codoping on the optical and scintillation performance of Cs3Cu2I5:Tl single crystals were studied. Cs3Cu2I5:Tl single crystals codoped with different concentrations of Sc3+, Y3+, and La3+ ions were grown by the Bridgman method. It was found that rare earth ion codoping reduces the optical transmittance of the as-grown crystals, in particular the coloring caused by Sc3+codoping. The luminescence mechanism associated with self-trapped exciton (STE) and Tl-related emissions has not been affected by the introduction of rare earth ion. The gamma scintillation yield and energy resolution are degraded after rare earth ion codoping.