Searching for inexpensive and earth-abundant photocatalysts with high activities has attracted considerable research in recent years. In this work, semimetallic tungsten diphosphide (WP₂) micro-particles are explored as a novel photocatalyst for the first time. The WP₂ particles were synthesized through a solid-state reaction route via vacuum encapsulation technique following by water washing. The first principle calculations and electric transport measurements show a semimetallic characteristic of WP₂, however a strong absorption in the UV range is observed. The prepared WP₂ particles exhibit an admirable photocatalytic activity towards oxidation of methyl orange and reduction of water for H₂ evolution with the assistance of co-catalyst element Pt under UV light irradiation. Hydroxyl radicals detected by fluorescence spectroscopy in the solution in the presence of WP₂ under UV light irradiation confirms the photoactivity. Furthermore, the photocatalyst shows a good photostability and reusability even after three successive experiment runs. Based on the experimental measurements and theoretical calculations, a possible photocatalytic mechanism is proposed for semimetallic WP₂. The present study may provide a chance for practical applications of the semimetallic material WP₂ in the field of photocatalysis.