We report on the high-performance nanoporous (NP) GaN-based metal-insulator-semiconductor (MIS) ultraviolet (UV) photodetectors (PDs) with a thermal oxidized β-Ga2O3insulating layer. The devices show a high responsivity of 4.5×105 A/W and maximum external quantum efficiency of 1.55×108% at 360 nm under a 10 V applied bias, which are attributed to the trap-assisted tunneling induced internal gain mechanism. Correspondingly, a specific detectivity of 8.27×1015 Jones and excellent optical switching repeatability are also observed in our fabricated PDs. The NP-GaN/β-Ga2O3 MIS UV PD may act as an excellent candidate for the application in UV photodetection due to the high performance and simple fabrication process.