Information on spatial and temporal variation in abundance is crucial for effective management of wildlife. Yet abundance estimates for the Critically Endangered Sumatran tiger Panthera tigris sumatrae are lacking from Riau, the province historically believed to hold the largest percentage of this subspecies. Recently, this area has had one of the highest global rates of deforestation. Using camera traps we investigated tiger abundance across peatland, flat lowland, and hilly lowland forest types in the province, and over time, in the newly established Tesso Nib National Park, central Sumatra. We estimated densities using spatially explicit capture-recapture, calculated with DENSITY, and traditional capture-recapture models, calculated with CAPTURE. With spatially explicit capture-recapture the lowest tiger density (0.34 +/- SE 0.24 per 100 km(2)) was estimated in the hilly lowland forest of Rimbang Baling and the highest (0.87 +/- SE 0.33 per 100 km(2)) in the flat lowland forest of the Park. Repeated surveys in the Park documented densities of 0.63 +/- SE 0.28 in 2005 to 0.87 +/- SE 0.33 per 100 km(2) in 2008. Compared to traditional capture-recapture the spatially explicit capture-recapture approach resulted in estimates 50% lower. Estimates of tiger density from this study were lower than most previous estimates in other parts of Sumatra. High levels of human activity in the area appear to limit tigers. The results of this study, which covered areas and habitat types not previously surveyed, are important for overall population estimates across the island, provide insight into the response of carnivores to habitat loss, and are relevant to the interventions needed to save the tiger.