Photoactive semiconductors as dye degradation catalysts play an important role in various industries. These materials rely on the external stimulus of photoirradiation to generate reactive oxygen species (ROS), which limits their practical application in real environments. Herein, simple ZnO nanocrystal coated zinc particles are proven to continuously generate ROS in the absence of an external energy input (in the dark). The core/shell structured Zn/ZnO showed an excellent dye degradation capability, which can degrade a series of dyes (cresol purple, methylene blue, methyl orange, rhodamine B, phenol red, and alizarin red) in water. Further studies showed that Zn/ZnO constantly generates ·O2– radicals and H2O2, which degrade dyes. The mechanism for the generation of ROS in the dark was proposed through zinc corrosion and electron transfer processes. This new material provides a potential solution to general wastewater treatment and drinking water purification.