An adjustable joint splint has a splint, two pads and an adjustable strap. The splint has two curved end segments and two connectors. The curved end segments are curved in the same direction and are parallel to each other, and each has an inner surface. The connectors connect the curved end segments. The pads are mounted respectively on the inner surfaces of the curved end segments. The adjustable strap is mounted between the connectors. Doctors can easily adjust the adjustable strap and the splint to conform to joints of different patients, patients can also tighten or loosen the strap around their joints themselves, and the splint has an opening that provides good ventilation. Therefore joints are held securely and comfortably by the splint, and skin in the area of the immobilized joint does not become irritated.