In this Letter, the physical layer security for a novel reconfigurable intelligent surface (RIS)‐assisted non‐orthogonal multiple access (NOMA) system in a multi‐user scenario is investigated, where the authors consider the worst case that the eavesdropper also utilises the advantage of the RISs. More specifically, they derive analytical results for the secrecy outage probability (SOP). From the numerical results, they observe that the use of RISs can improve the secrecy performance compared to traditional NOMA systems. However, for the worst case that the received signals at the eavesdropper come from the RISs and source, increasing the number of intelligent elements on the RIS has a negative impact on the secrecy performance. At high signal‐to‐noise ratios, the system's SOP tends to a constant. Finally, the secrecy performance can be improved through group selection.