Methane (CH4) is a potent greenhouse gas, yet its future air-sea diffusive fluxes under future climate scenarios remain highly uncertain due to the limited observational coverage. Here, we present a multi-model assessment of surface ocean CH4 concentrations and air–sea diffusive fluxes from 1850 to 2100 using four CMIP6 Earth system models and a Random Regression Forest (RRF) trained with a newly compiled observational database comprising more than 280,000 CH4 measurements. We estimate the global oceanic CH4 diffusive flux during 1980–2014 to be 2.6 ± 0.8 Tg CH₄ yr⁻¹, representing a reduction of approximately 79% since 1850, largely driven by increasing atmospheric CH4 concentrations. As atmospheric CH4 continues to rise under SSP370 and SSP585, all four CMIP6 models consistently project a regime shift during the 21st century, with the ocean transitioning from a net source to a net sink of CH4. This projected reversal carries profound implications for the global CH4 budget and future climate change, potentially redefining the ocean’s role in regulating atmospheric CH4.