The 7.5 earthquake that struck central Papua New Guinea in 2018 was the largest event ever recorded in the region with modern seismic instruments. The ground motions associated with this event also triggered widespread landslides and affected more than 500,000 people. However, due to the absence of a local seismic and Global Positioning System network in the vicinity, the fault location, system, and slip distribution of this earthquake are not well documented. In this study, we use the subpixel offset method on the Copernicus Sentinel‐1 Synthetic Aperture Radar (SAR) images to calculate the 3D coseismic displacement of the 2018 Papua New Guinea earthquake. The results show clear fault traces that suggest coseismic slip on the Mubi fault and the Mananda fault that triggered landslides that spread out in a more than region. Finite‐source inversions based on the subpixel offset measurements show up to 4.1 and 6.5 m coseismic slip on the Mubi and Mananda faults, respectively. Despite higher data uncertainty () of the subpixel offset data, synthetic resolution tests show resolvable slip above 8 km in depth. The lack of shallower slip on the west side of the Mananda fault could be due to an inflated geothermal gradient near the dormant volcano, Mount Sisa, as a slip barrier. The result of the coulomb stress change suggests possible southeastward slip propagation from the Mananda fault to the Mubi fault. Our work successfully resolves 3D coseismic displacement in highly vegetated terrains and demonstrates the feasibility of using the subpixel offset on SAR images to help our understanding of regional active tectonic systems.