The Reunion harrier is an endangered raptor that is endemic to Reunion Island. Anticoagulant rodenticides (ARs) are widely applied on the island to prevent leptospirosis transmission to humans and limit the damage to sugarcane crops caused by rats and house mice. As these pesticides exhibit a high risk of secondary poisoning for rodent predators, we examined whether the Reunion harriers were exposed to and potentially poisoned by ARs. The AR concentrations in the livers of 58 harrier carcasses collected from 1999 to 2016 were measured. Both the temporal and spatial trends were analysed, and the influences of individual and landscape characteristics on the liver concentrations and potential poisoning were determined. AR residues were detected in 93% of the harriers. Difenacoum was the most frequently found (73% of positive races), while brodifacoum and bromadiolone showed the highest concentrations. Both the numbers of harriers exposed to ARs and of individuals that were potentially poisoned increased over time. This is particularly alarming as the number of harriers potentially poisoned by brodifacoum increased dramatically beginning in 2014. We also showed that the landscape composition of the townships influenced the AR exposure, as the concentrations increased with the proportion of urban areas and showed a peak at 25% of the township under sugarcane cultivation. We conclude that AR poisoning is likely a main threat for Reunion harrier conservation and propose several actions to limit poisoning.