Self-powered photodetectors have demonstrated potential for developing future wireless and implantable devices. Herein, we present a self-powered UV photodetector with an ultrahigh photoresponse based on vertically oriented and high crystalline quality n-type GaN nanorod arrays: poly(methyl methacrylate)/p-Si heterojunction. Benefiting from the highly efficient separation and transport of photoexcited electron-hole pairs, significant improvements in photoresponsivity are experimentally obtained. In a zero-biased self-powered detection mode, a 6.7AW-1 responsivity and 2.68×1013 Jones detectivity are achieved under 355 nm light illumination, and the response time is as low as 0.29/3.07 ms (rise/fall times).