The endangered iguana Cyclura cornuta stejnegeri is endemic to Mona Island, Puerto Rico. Factors accounting for its protected status include low adult densities and relatively low abundance of juveniles. We studied the spatial ecology, territoriality, and philopatric behavior along a gradient of human-disturbed areas to increase our understanding of its unusual demography. We used the minimum convex polygon (MCP) method to estimate the home range of radiomarked iguanas, the degree of inter- and intrasex home-range overlap, and the temporal patterns of space use by 42 iguanas (19 males and 23 females, including 4 juvenile individuals) during the nonbreeding (October–November) and mating season (June) at three Mona Island localities with different levels of human disturbance. Juvenile iguanas had larger home ranges and move across the home ranges of several adults. Adult males had larger home ranges than females and no significant temporal differences in size or pattern of usage (Cole index) were detected between the study periods. Females, despite reducing their home-range size during the mating season, changed the usage patterns, resulting in increased interactions with neighboring males. Home-range overlap was minimal between males, followed by female–female overlap and maximal between males and females. Our results suggest that Mona Island iguanas are highly territorial throughout the year; particularly males in the mid- and undisturbed areas where almost all home-range areas appear to be equivalent to the defended territory. In the most disturbed area, where supplemental feeding by humans exists, home ranges are smaller and there is larger intersex home-range overlap. Previously, the low density of the population was attributed to lack of recruitment into adult stages due to predation of juveniles by nonnative mammals. However, the high levels of territoriality documented in this study may be an additional factor explaining the low densities exhibited by this population. Our data suggest that the three study sites may be at or close to carrying capacities for males, because there appear to be no vacant areas for additional males. Furthermore, the large and highly overlapping home ranges exhibited by four juvenile iguanas suggest that they are or will become floaters in search of unoccupied space.