The Weddell seal is a fast-ice-obligate, mesopredator found around the Antarctic coastline. Traditional methods of understanding population abundance, including shipboard or aerial transects, are cost-prohibitive and risky, so we combined high-resolution satellite imagery (VHR) with the web platform, Tomnod, to conduct a citizen science campaign to: 1) determine where seals are present, 2) estimate abundance where present and 3) understand environmental variables that influence habitat preference. We produced the first-ever population estimate for this species, determining that the global population of Weddell seals during 2011 was c. 40% of the ~800,000 animals previously estimated. Fast-ice conditions, ocean depth and proximity to Adélie and emperor penguins were related to seal presence, though varying among Antarctic regions. We developed a reliable method to monitor populations at regional and global scales, along with models that could be used to assist in ecologically sound management.