Armed conflict, and its end, can have powerful effects on natural resources, but the influence of war and peace on highly biodiverse tropical forests remains disputed. We found a sixfold increase in fires in protected areas across biodiversity hotspots following guerrilla demobilization in Colombia, and a 52% increase in the probability of per-pixel deforestation within parks for 2018. Peace requires urgent shifts to include real-time forest monitoring, expand programmes to pay for ecosystem services at the frontier, integrate demobilized armed groups as staff of protected areas, and establish a domestic market for frontier deforestation permits.