Campi Flegrei is an active caldera in Southern Italy and one of the most hazardous volcanoes in the world. Here, we provide the first comprehensive 3D geophysical image of its plumbing system down to 50 km using the seismic receiver function technique. We imaged major crustal and upper mantle structures and located seismic discontinuities, identifying two significant low-velocity zones. A shallow LVZ, in the eastern part of Campi Flegrei caldera at 10 km of depth, has a vertical prolate ellipsoid shape (6 × 5 × 4 km), suggesting a localized crustal magma transfer or intrusion zone. A deeper LVZ, dipping northward, is 17 km thick, with a top at 25 km of depth and up to 30 wt% melt, indicating a subcrustal primitive magma source. These findings agree with previous petrological studies and enhance our understanding of the Campi Flegrei magmatic system, its ongoing activity, unrest, and associated hazards.