We present an ultrasensitive ultraviolet (UV) detector based on a p-type ZnS nanoribbon (NR)/indium tin oxide (ITO) Schottky barrier diode (SBD). The device exhibits a pseudo-photovoltaic behavior which can allow the SBD to detect UV light irradiation with incident power of 6 × 10^-17 W (~85 photons/s on the NR) at room temperature, with excellent reproducibility and stability. The corresponding detectivity and photoconductive gain are calculated to be 3.1 × 10²⁰ cm·Hz^1/2·W^-1 and 6.6 × 10⁵, respectively. It is found that the presence of the trapping states at the p-ZnS NR/ITO interface plays a crucial role in determining the ultrahigh sensitivity of this nanoSBDs. Based on our theoretical calculation, even ultra-low photon fluxes on the order of several tens of photons could induce a significant change in interface potential and consequently cause a large photocurrent variation. The present study provides new opportunities for developing high-performance optoelectronic devices in the future.