An embodiment rear floor side member assembly includes a rear side member including a rear inner portion disposed toward an inside of a vehicle body, a rear upper portion curved toward an outside of the vehicle body from the rear inner portion, and a rear lower portion curved from the rear inner portion toward the outside of the vehicle body, a reinforcement mounting portion formed on the rear lower portion, a rear side member reinforcement mounted on the reinforcement mounting portion, and a rear spring seat assembly mounted at a bottom part of the rear side member reinforcement.