The Coahuila box turtle (Terrapene coahuila) is an endangered species of chelonian endemic to the Cuatro Ciénegas valley in northern Mexico. It is the only aquatic member of the genus Terrapene and is dependent on permanent and seasonal wetlands. Over the past several decades, T. coahuila populations have declined from habitat loss as the wetlands have dried due to human modification of the valley. We conducted a survey of the status of the species from 2011 to 2018, updating previous estimates of population density and overall population size. We also collected data on sex ratio in each of the 8 wetland study areas in the valley and report a strongly male-biased sex ratio. Our results indicate a total population size of approximately 1791 individuals, based on recorded densities from 0.24 to 3.3 individuals/ha among 539.76 ha of suitable habitat in the 8 wetland areas. This estimate is lower than previous studies indicated, implying direct effects of habitat loss on T. coahuila. If habitat loss due to lowering of the water table continues, this species will become extinct. We recommend conservation measures including upgrading the species' International Union for Conservation of Nature Red List status to Critically Endangered, protecting and restoring key wetlands in the valley, and establishing captive assurance colonies in Mexico.