Global methane (CH4) emissions have reached approximately 600 Tg per year, 20%–40% of which are from wetlands. Of the primary factors affecting these emissions, the water table level is among the most uncertain. Here we conduct a global meta-analysis of chamber and flux-tower observations of CH4 emissions and employ a novel mechanistic model to show that wetlands have maximum emissions at a critical level of inundation and discuss its origin. This maximum arises from an interplay between methanogenesis, methanotrophy, and transport, whose rates vary differently with the inundation level. The specific location of the critical water level above the soil surface may differ depending on wetland characteristics, for example temperature or the presence of macrophytes with aerenchyma. However, data suggest that globally a water level of about 50 cm is the most favorable to CH4 emissions. Keeping the water level away from this critical value could reduce methane emissions in human-made wetlands, which comprise at least one fifth of the global wetland area.