We present the first systematic assessment of the population, demography and distribution of the Endangered Zanzibar red colobus Piliocolobus kirkii, in Unguja in the Zanzibar archipelago, based on a survey effort of 4,725 hours. We estimate the total population comprises 5,862 individuals in 342 groups (mean group size 17.12); 3.4 times the mean of all previous estimates. We calculated a total area of occupancy of 376 km2, with 4,042 individuals living within protected areas. Mean group sizes were significantly higher within protected areas (20.57) than outside (12.80). The number of adult females was 3,179 (54.21%), with a mean of 9.29 per group, and the number of adult males was 932 (15.89%), with a mean of 2.71 per group, giving a ratio of 3.31 adult females to adult males. This ratio was significantly lower outside protected areas. The total number of infants was 958 (16.34%), with a mean of 2.80 per group, and the number of subadults/juveniles was 793 (13.52%), with a mean of 2.32 per group, giving ratios of 0.30 infants to adult females, and 0.25 subadults/juveniles to adult females. The results indicate that P. kirkii is resilient and thriving far better than assumed. However, recruitment is low and the population may be in decline, with individuals outside protected areas most at risk. We tentatively support the categorization of P. kirkii as Endangered on the IUCN Red List, argue for greater protected area status for southern Uzi, Vundwe and Mchamgamle, and discuss conservation implications for this charismatic flagship species.