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Abstract I investigate the detectability of magma reservoirs in the vertical gravity gradient (VGG) anomalies calculated from satellite altimetry data. First, I calculate a suite of synthetic seamount models to show the expected VGG anomaly characteristic wavelength and amplitude for a simplified magmatic system, hydrothermal system, and a caldera infill, varying their dimensions for a given depth and density contrast. I find that most magmatic and hydrothermal systems create VGG anomalies with a characteristic wavelength and amplitude greater than the data uncertainty and are therefore detectable. The proposed approach consists in three main steps: (a) calculate the VGG from the two components of the deflection of the vertical, (b) calculate and remove the gravity contribution of the bathymetry interface using an independent bathymetry data set (e.g., acquired by multibeam echosounders) to obtain a VGG Bouguer gravity anomaly, (c) invert the Bouguer VGG anomaly to obtain a 3D density model. I image a 6‐by‐8‐km low density body between 3 and 9 km depth under Brothers volcano in the Kermadec arc. I hypothesize that it represents the main magmatic system, possibly with a minor fraction of hydrothermal fluids at the shallower depths. There are about 225 submarine volcanoes globally that could be studied with satellite altimetry‐derived gravity data to potentially image their magmatic system. Future altimetry data will increase the gravity data resolution and allow us to image smaller features. This is thus an invaluable data set for the study of underexplored submarine volcanoes and can help improve our volcano hazards assessment.