We conducted a study on the morphological variation and adaptation of semi-wild provisioned Macaca mulatta brevicaudus in the Nanwan Nature Reserve for Rhesus Macaques, Hainan, China. We measured the body weights of 253 individuals, and recorded various length morphometrics (head, crown-rump, tail, fore foot, back foot, forearm, post-brachium, thigh and crus) from 132 individuals. We compared these measurements with those recorded from other Macaca mulatta subspecies, and also investigated the correlation between body weight and social dominance. Our results show the following results. M. m. brevicaudus is significantly smaller than other mainland populations of rhesus macaque subspecies, which could be explained by Bergmann’s rule and the island rule. Males grow faster and attain larger adult body sizes than females. There is no evidence for between-group variation in body sizes indicating that there is a low-level of intergroup competition in this subspecies. Higher dominance rank is positively associated with certain morphological features (body weight, crown-rump length and head length), but has no significant relationship with others (tail length, forearm length, thigh length and foot length). This paper provided basic morphological data for a wild population of a rhesus macaque subspecies, and will be useful in understanding ecological adaptation and promoting population management of this primate species.