A supplemental restraint system that is mountable to the frame of a stationary structure, such as a police station or other building, to secure a prisoner in a seated position. The supplemental restraint system includes a restraint bar that is pivotally supported on the mounting plate for movement between a raised inoperative position and a lowered operative position. A locking mechanism secures the restraint bar in any selected lowered position and includes an actuation lever for selectively releasing the locking mechanism to permit the restraint bar to be raised for release of the prisoner. A spring mechanism biases the restraint bar toward a raised position so that the restraint bar can be moved upwardly while the police officer is outside of the police vehicle. Mounting plates can be utilized to secure the supplemental restraint system to the floor structure and an adjacent wall structure.