Near-bottom magnetic field data were collected using a towed magnetometer over selected parts of Palinuro and Marsili submarine volcanoes in the southern Tyrrhenian Sea, Italy. We obtained equivalent magnetizations maps at these sites by inverting the corresponding magnetic anomalies, highlighting the seafloor expression of hydrothermal alteration. Zones of reduced magnetization are interpreted as evidence for alteration related to hydrothermal processes; they are associated with water-column and seafloor observations of hydrothermal activity and altered host rocks. At Marsili volcano, a large elliptical area of reduced magnetization is located south of the summit cone and perpendicular to the trend of Marsili volcano itself. This confirms the presence of a large hydrothermal system centered over the more recent eastern volcanic ridge and its associated magma chamber. Similarly, areas of reduced magnetization over Palinuro caldera are consistent with hydrothermal venting occurring along the caldera walls, consistent with permeability structures related to caldera ring faults providing preferred pathways for the upflow of hydrothermal fluids.