Inland waters are the largest natural source of methane (CH4), a potent greenhouse gas, but models and estimates of aquatic CH4 cycling and emissions were developed in soft-water ecosystems and may not apply to globally abundant salt-rich inland waters. Here we show that elevated salinity constrains microbial CH4 cycling restricting aquatic emissions at large scales. Our survey of the Canadian Prairie ecozone demonstrates that salinity interacts with organic matter availability to shape CH4 patterns across aquatic networks (rivers, lakes, wetlands, and agricultural ponds). Current empirical models, biased toward solute-poor surface waters, overestimated CH4 concentrations and emissions measured in hardwater systems by up to several orders of magnitude, with discrepancies strongly linked to salinity. Models were particularly inaccurate for bubble-mediated emissions from small lentic systems, one of the largest sources of aquatic CH4 globally. Elevated salinity reduced aquatic CH4 emissions by an estimated 81 % in the Canadian Prairies, and could result in a 7.8 % overestimation of global lentic emissions. Widespread salinization of inland waters under future land use and climate regimes could further restrict methane emissions.