<p>Methane is a potent greenhouse gas, and aquatic environments such as wetlands, lakes, rivers, and estuaries are recognized as significant natural sources of methane emissions. However, a major gap in current methane budgets is the limited understanding of methane contributions from groundwater–surface water interactions. Groundwater discharge has often been overlooked in methane budgets due to the difficulty of detecting and quantifying both discharge rates and the associated methane fluxes. This study addresses this gap by synthesizing global literature to estimate groundwater methane concentrations and fluxes across various aquatic environments. A systematic review of peer-reviewed literature from 2011 to 2023 was conducted, synthesizing data on groundwater methane concentrations, discharge rates, and environmental factors across diverse aquatic environments. These data were subsequently integrated with global groundwater discharge estimates to quantify methane fluxes, thereby advancing current understanding of groundwater contributions to aquatic methane emissions and their potential role in the global carbon cycle. The main findings are as follows: (1) The average global groundwater methane concentration was 0.31 ± 0.47 mmol/L. (2) The global average groundwater-driven methane flux was estimated to be 3.9 ± 6.2 mmol/m²/day. (3) When extrapolated globally using existing estimates of groundwater discharge, these fluxes suggest that groundwater may account for up to 70% of methane emissions from surface waters in aquatic systems. (4) In some environments, groundwater-driven methane fluxes exceed direct surface water–atmosphere methane emissions by several fold, particularly in subarctic and temperate regions. This significant groundwater contribution has been largely unrecognized in global methane budgets and climate models. Findings indicate a lack of studies quantifying groundwater-driven methane fluxes (or providing concurrent data on groundwater methane concentrations and groundwater discharge), particularly in streams. Incorporating groundwater-driven methane into carbon accounting frameworks is essential for improving the accuracy of methane emission estimates and for better informing climate mitigation strategies. Further empirical studies are needed to reduce uncertainty and address current data gaps across geographic regions and aquifer types.</p> Graphical Abstract <p></p> <p>Methane, a potent greenhouse gas, is emitted directly to the atmosphere from aquatic environments like wetlands, lakes, rivers, and estuaries, yet the contribution from groundwater discharge to these emissions remain poorly understood, a gap this study addresses. The graphical abstract illustrates the formation of methane in anaerobic groundwater systems, followed by its discharge into aquatic environments, and release to the atmosphere. Arrows link these hydrogeological and biogeochemical processes. This study, synthesizing global literature from 2011 to 2023, estimates groundwater methane concentrations and fluxes, revealing a global average groundwater methane concentration of 0.31 ± 0.47 mmol/L and a global groundwater driven methane flux of 3.9 ± 6.2 mmol/m²/day. A key finding is that groundwater discharge may contribute up to 70% of methane emissions from aquatic environments, with fluxes exceeding surface water emissions by up to 6 times, especially in subarctic and temperate regions. The role of groundwater discharge in methane budgets has been largely unrecognized, while this study clearly shows the need for its integration into carbon accounting frameworks.</p>