Ammonia is a critical chemical in agriculture and industry that is produced on a massive scale via the Haber–Bosch process¹. The environmental impact of this process, which uses methane as a fuel and feedstock for hydrogen, has motivated the need for more sustainable ammonia production^2,3,4,5. However, many strategies that use renewable hydrogen are not compatible with existing methods for ammonia separation^6,7,8,9. Given their high surface areas and structural and chemical versatility, metal–organic frameworks (MOFs) hold promise for ammonia separations, but most MOFs bind ammonia irreversibly or degrade on exposure to this corrosive gas^10,11. Here we report a tunable three-dimensional framework that reversibly binds ammonia by cooperative insertion into its metal–carboxylate bonds to form a dense, one-dimensional coordination polymer. This unusual adsorption mechanism provides considerable intrinsic thermal management¹², and, at high pressures and temperatures, cooperative ammonia uptake gives rise to large working capacities. The threshold pressure for ammonia adsorption can further be tuned by almost five orders of magnitude through simple synthetic modifications, pointing to a broader strategy for the development of energy-efficient ammonia adsorbents.