To avoid the risk of misapplying conservation effort the correct diagnosis of the agent causing a population to decline requires scientific approaches. The radiated tortoise Geochelone radiata, endemic to southern Madagascar, is heavily harvested for food and for the pet trade. Fearing overexploitation, the tortoise was protected under Malagasy law and placed on Appendix I of CITES, yet scientific evidence that the radiated tortoise is declining, and that exploitation is the agent driving any decline, is lacking. Interviews with tortoise harvesters, a comparison of the size of the tortoise's range through time, and estimates of tortoise abundance at 14 sites under different levels of harvest intensity were used to seek evidence of overexploitation. In the first study to attempt to quantify the size of the illegal harvest of radiated tortoises, we estimated that up to 45,000 adult radiated tortoises are harvested each year. The species is declining, with its range having contracted by one-fifth over the last 25 years. Three pieces of evidence strongly suggest that overexploitation is driving this decline. Firstly, commercial harvesters reported travelling increasingly far, up to 200 km, to find sufficient densities of tortoises. Secondly, tortoises were either absent or at very low abundance at sites subject to commercial harvesting, but in remote, unharvested regions, tortoises persisted at densities of up to 2,500 km−2. Thirdly, tortoise abundance increased significantly with distance from urban centres of high demand for tortoise meat. If current rates of harvesting continue, the radiated tortoise will go extinct in the wild.