About 80% of Earth volcanic activity occurs underwater, releasing deep carbon to submarine environments and impacting Earth’s climate over geological timescales. The CO2 emitted during submarine eruptions and/or hydrothermal degassing creates local ocean acidification, affecting the seawater carbonate equilibrium and oceanic ecosystems at large regional scales. Here, we report for the first time the existence of a major CO2 hydrates field at the seafloor offshore Mayotte Island (Indian Ocean) associated with liquid CO2 venting, following the submarine eruption that occurred in 2018. Using detailed acoustic surveys and in situ Raman spectroscopy, we reveal multiple hydrate mounds and seep zones distributed over an area of 0.06 km². We show that the gas seeps are mainly composed of CO2, with minor contributions of CH4 and H2, with noble gas ratios and stable and radio-carbon isotopes clearly demonstrating their magmatic origin. Estimates of the CO2 emitted over the entire area represent about 0.5% of the global magmatic carbon flux. Our discovery also suggests that CO2 hydrates may potentially be stable at the seafloor at the right pressure-temperature conditions, bringing new prospects into CO2 sequestration and decarbonization pathways in the ocean, in particular regarding kinetics of hydrates dissolution and environmental impacts.