Cooperative rate splitting (CRS), built upon rate splitting multiple access (RSMA) and opportunistic user relaying, has been recognized as a promising transmission strategy to enhance the user fairness and spectral efficiency in multiantenna broadcast channels. To further boost its performance, the interplay of CRS and reconfigurable intelligent surface (RIS) is investigated in this work. Specifically, a novel RIS-aided CRS transmission framework is proposed and the corresponding resource allocation problem to maximize the minimum rate among users is investigated. An alternative optimization algorithm is then proposed to optimize the transmit beamforming, common rate allocation, and RIS phases, iteratively. Numerical results show that the proposed RIS-aided CRS transmission framework significantly improves the spectral efficiency compared with its non-cooperative counterpart and other schemes without RIS.