In general, geothermal systems in Indonesia are characterized by the manifestation of fluid at the surface with boiling temperatures, which occur in the area of Quaternary and active volcanoes along the volcanic arcs. All Quaternary volcanic regions are related to cooling magma or igneous rock intrusion, which in turn is a source of heat from the geothermal system in the active arc (Hochstein and Sudarman, 2008). Ambon is in the southern part of Seram Island, which belongs to the outer Banda arc (Figure 1). The Banda arc is mostly composed of mélange rocks as a result of Tertiary subduction (Marini and Susangkyono, 1999). From this process, Ambon and the surrounding islands have many geothermal manifestations that appear on the surface (Poorter et al., 1989). Several studies on the Banda arc in eastern Indonesia have been carried out previously. Milsom et al. (2001) qualitatively reviewed the short-wavelength high gravity anomalies around the Banda Sea caused by Ophiolite rock dominance. Widiwijayanti et al. (2004) conducted studies on the evolution of geodynamics in the Maluku sea based on three-dimensional (3D) inversion of the Earth's gravitational field. Fichtner et al. (2010) studied lithosphere layer subduction in the Seram Sea using a combination of full-waveform tomography and isotope ratios. Widiyantoro et al. (2011) determined a complex structural model of the lithosphere slab under the Banda arc using a seismic Abstract: Ambon and Seram islands in eastern Indonesia are located in the convergence zone of three major plates, causing this area to have many geothermal manifestations on the surface. A subsurface geological study on Ambon and Seram has been conducted based on the integration of gravity and magnetic inversion modeling to determine the intrusion of igneous rock as a potential source of geothermal energy. The gravity and magnetic inversion utilized gravity and magnetic anomaly data from the World Gravity Map (WGM) and the Earth Magnetic Anomaly Grid (EMAG), as well as the CRUST 1.0 global earth crust model, as the constraints. In general, gravity and magnetic anomalies in the study area are compatible, in which the southern part of Ambon is dominated by high anomalies and the northern part is covered by low anomalies associated with the Seram trench. The high anomaly is thought to be related to the intrusion of igneous rocks underneath the surface that have cooling and are rich in magnetic minerals, while the low anomaly is related to the destruction zone due to subduction in the north of Seram. The gravity inversion shows the distribution of subsurface rock density ranging from 1.86 g cm-3 to 2.82 g cm-3 , while magnetic inversion produces rock susceptibility contrast ranging from-0.0034 SI to 0.0058 SI. Based on Curie point depth analysis of magnetic data, the bottom depth of the magnetic source is estimated at 25 km. The 3D subsurface model of Ambon and Seram based on gravity and magnetic inversion shows a massive and evident intrusion pattern with high rock density and susceptibility contra...