Abstract: A spring-biased mechanism for a trailer coupler to positively connect a trailer to a towing vehicle. The coupler is attached to a tongue of a trailer and adapted to receive a hitch ball of a towing vehicle. A locking assembly mounted within the coupler prevents withdrawal of the hitch ball from the coupler housing to prevent inadvertent separation of the trailer from the towing vehicle. The lock includes a rotatable pin extending transversely through the coupler housing to selectively prevent withdrawal of the hitch ball. The rotatable pin is connected to a spring-biased lever positioned externally of the coupler housing. In addition to pivoting between a locked and an open position to rotate the pin, the lever can be laterally shifted to be held open against the biasing of the spring allowing insertion of the ball. To lock onto the hitch ball, the lever can be pulled outwardly causing the spring to bias the lever to the locked position.