Limited space on islands usually cannot sustain stable populations of large predators. However, jaguars, the largest cat species in the Americas, unexpectedly occur in the Estação Ecológica Maracá-Jipioca reserve, a system of continental Amazonian islands in the Atlantic Ocean. We investigated jaguar population structure, density, and activity patterns. We placed 25 camera-trap stations across 149.19 km² and used spatially explicit capture–recapture to estimate density, and Rayleigh's test to assess activity patterns. We identified 21 individuals (12 females, six males, and three cubs) and estimated an adult density of 6.7 individuals per 100 km², which equals a population of approximately 43 jaguars. The population is composed mostly of females (66%) in relation to males (33%). Male and female activity patterns overlapped and showed more activity during daytime. The high jaguar density and the presence of females with cubs indicate that these islands are likely natural refugia for jaguars, reinforcing the importance of this protected area for jaguar conservation and possibly challenging the paradigm that large-mammal populations are not feasible in restricted islands.