Abstract: A hat size adjustment system which has a pair of size adjusting straps with free ends that can be tucked inside slots. Each strap has one end secured to the hat on opposite sides of an opening in the back of the crown. One strap is secured to a sweatband inside the crown. The other strap is secured to an inside lower edge of the crown. The straps are releasably secured to each other by hook and loop fasteners. The free ends of the straps are tucked into slots on each side of the crown opening to present a clean and attractive looking hat size adjustment system. When properly fastened, the size adjustment system appears to be a single strap fixed on both ends.