Salinity significantly restricts methane (CH₄) emissions from small inland waters, particularly in salt-rich regions like the Canadian Prairies. This study combines field surveys and eddy covariance measurements to show that salinity limits microbial CH₄ cycling through complex mechanisms, reducing emissions from one of the largest global hardwater regions. Existing models overestimated CH₄ emissions from ponds and wetlands by up to several orders of magnitude, with discrepancies linked to salinity. While not significant for rivers and larger lakes, salinity interacts with organic matter availability to shape CH₄ patterns in small lentic habitats. The study estimates that excluding salinity leads to overestimation of emissions from small Canadian Prairie waterbodies by at least 81% (≈1 Tg CO₂ equivalent), a quantity comparable to other major national emissions sources. The findings are consistent with patterns in other hardwater landscapes, likely leading to an overestimation of global lentic CH₄ emissions. Widespread salinization of inland waters may impact CH₄ cycling and should be considered in future projections of aquatic emissions. Salinity affects microbial communities, particularly methanogens and methanotrophs, and inhibits CH₄ production through multiple mechanisms. Sulfate (SO₄²⁻) and iron (Fe³⁺) reducers outcompete methanogens for labile C substrates. Salinity also interacts with other key CH₄ controls, such as organic C availability and nutrient availability, to suppress CH₄ concentrations. The study surveyed 193 aquatic ecosystems in the Canadian Prairies, including rivers, lakes, wetlands, and agricultural ponds, and found that salinity restricts CH₄ emissions, especially through ebullition. The results show that salinity is a key driver of CH₄, interacting with organic matter content to shape surface CH₄ partial pressure (pCH₄). The findings are consistent with data from other global hardwater landscapes, suggesting that salinity likely downregulates emissions worldwide. The study tested existing empirical models and found that they overestimated CH₄ emissions from salt-rich inland waters. Including salinity as a model term significantly improves the accuracy of predictions. The study also estimated that accounting for salinity reduces the emissions budget for the Canadian Prairies by up to 81%, which is comparable to other major national emissions sources. Globally, using soft-water-derived models to estimate hardwater CH₄ fluxes can lead to overestimations, with consequences for planetary budgets. The study highlights the importance of incorporating salinity in emissions models to improve accuracy and reduce overestimations. Future salinization may further reduce aquatic CH₄ emissions, and the study emphasizes the need to consider salinity in future emissions scenarios.Salinity significantly restricts methane (CH₄) emissions from small inland waters, particularly in salt-rich regions like the Canadian Prairies. This study combines field surveys and eddy covariance measurements to show that salinity limits microbial CH₄ cycling through complex mechanisms, reducing emissions from one of the largest global hardwater regions. Existing models overestimated CH₄ emissions from ponds and wetlands by up to several orders of magnitude, with discrepancies linked to salinity. While not significant for rivers and larger lakes, salinity interacts with organic matter availability to shape CH₄ patterns in small lentic habitats. The study estimates that excluding salinity leads to overestimation of emissions from small Canadian Prairie waterbodies by at least 81% (≈1 Tg CO₂ equivalent), a quantity comparable to other major national emissions sources. The findings are consistent with patterns in other hardwater landscapes, likely leading to an overestimation of global lentic CH₄ emissions. Widespread salinization of inland waters may impact CH₄ cycling and should be considered in future projections of aquatic emissions. Salinity affects microbial communities, particularly methanogens and methanotrophs, and inhibits CH₄ production through multiple mechanisms. Sulfate (SO₄²⁻) and iron (Fe³⁺) reducers outcompete methanogens for labile C substrates. Salinity also interacts with other key CH₄ controls, such as organic C availability and nutrient availability, to suppress CH₄ concentrations. The study surveyed 193 aquatic ecosystems in the Canadian Prairies, including rivers, lakes, wetlands, and agricultural ponds, and found that salinity restricts CH₄ emissions, especially through ebullition. The results show that salinity is a key driver of CH₄, interacting with organic matter content to shape surface CH₄ partial pressure (pCH₄). The findings are consistent with data from other global hardwater landscapes, suggesting that salinity likely downregulates emissions worldwide. The study tested existing empirical models and found that they overestimated CH₄ emissions from salt-rich inland waters. Including salinity as a model term significantly improves the accuracy of predictions. The study also estimated that accounting for salinity reduces the emissions budget for the Canadian Prairies by up to 81%, which is comparable to other major national emissions sources. Globally, using soft-water-derived models to estimate hardwater CH₄ fluxes can lead to overestimations, with consequences for planetary budgets. The study highlights the importance of incorporating salinity in emissions models to improve accuracy and reduce overestimations. Future salinization may further reduce aquatic CH₄ emissions, and the study emphasizes the need to consider salinity in future emissions scenarios.