A Mw 5.2 earthquake struck southeastern Java Island, Indonesia, on July 9, 2022, followed by more than 200 aftershocks over five days, including a Mw 5.0 event. Through integrated analysis combining an updated 1-D velocity model, array-based depth determination, double-difference relocation, and Bayesian moment tensor inversion, we reveal evidence for a previously unidentified active backthrust fault system in the southeastern Java forearc region. The mainshock occurred at approximately 20 km depth, with the relocated hypocenters forming a distinct south-dipping alignment situated 5–20 km above the subducting slab. Focal mechanisms consistently show thrust faulting with east–west strike orientation and steep south-dipping nodal planes—opposite to the subduction direction. This newly identified backthrust system is located approximately 150 km north of the trench line, with estimated fault dimensions of 20 km in length and 30 km in width. Coulomb stress modeling suggests the 1994 Mw 7.8 Banyuwangi earthquake may have contributed to the activation of this structure through a modest stress increase of approximately 1 bar. The identification of this backthrust system enhances our understanding of the complex deformation patterns in southeastern Java, and carries significant implications for regional seismic and tsunami hazard assessment. © 2025 Elsevier Ltd