Pyrethroid contact insecticides are mainstays of malaria control, but their efficacies are declining due to widespread insecticide resistance in Anopheles mosquito populations, a major public health challenge. Several strategies have been proposed to overcome this challenge, including insecticides with new modes of action. New insecticides, however, can be expensive to implement in low-income countries. Here, we report a simple and inexpensive method to improve the efficacy of deltamethrin, the most active and most commonly used pyrethroid, by more than 10 times against Anopheles mosquitoes. Upon heating for only a few minutes, the commercially available deltamethrin crystals, form I, melt and crystallize upon cooling into a polymorph, form II, which is much faster acting against fruit flies and mosquitoes. Epidemiological modeling suggests that the use of form II in indoor residual spraying in place of form I would significantly suppress malaria transmission, even in the presence of high levels of resistance. The simple preparation of form II, coupled with its kinetic stability and markedly higher efficacy, argues that form II can provide a powerful, timely, and affordable malaria control solution for low-income countries that are losing protection in the face of worldwide pyrethroid resistance. Significance The use of deltamethrin, a crystalline contact insecticide and a leading tool in combatting malaria vectors, faces an uncertain future, threatened by developing resistance of mosquitoes. A more active crystalline polymorph of deltamethrin, discovered here, speeds the knockdown of susceptible mosquitoes by a factor of up to 12 compared with the currently used crystalline form. The faster-acting deltamethrin polymorph is predicted to suppress malaria transmission and associated human mortality while reducing environmental exposure because less agent is required to achieve the same effect. The outstanding performance of form II promises increased serviceable use of deltamethrin crystals for indoor residual spraying. Metastable forms of contact insecticides should be considered generally for public health applications.