A retractable strap system for a roll up door of a truck featuring a housing with a spindle and a spring-loaded reel and a strap wrapped around the spindle. The spring-loaded reel is biased to retract the strap into the housing. A catch lever allows and prevents the spring-loaded reel from retracting of the strap into the housing. When the truck door is open, the strap can be pulled to a desired length and held at that desired length. The strap automatically retracts into a housing when the roll up door of the truck is closed, preventing its exposure or unnecessary wear.