A supplemental prisoner restraint system is mountable to the frame of a police car to secure a prisoner in a seated position. The supplemental restraint system includes a mounting plate that is secured to the frame, a restraint bar that is pivotally supported on the mounting plate for movement between a raised inoperative position and a lowered operative position, and a locking mechanism that secures the restraint bar in any selected lowered position. An actuation lever selectively releases the locking mechanism to raise the restraint bar 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. The supplemental restraint system can also be utilized in an office environment in conjunction with a seating device to temporarily secure a prisoner while being processed.