Our ecology is being threatened by electronic garbage. It takes time and effort to handle and recycle electronic trash. Researchers are trying to improve the existing recycling methods for a more effective process which produce fewer by-products. The present study is to develop a simple and practical hydrometallurgical process for producing copper oxide nanoparticles (CuO NPs) from waste printed circuit board (WPCB). WPCBs are selectively leached using aqueous ammonium salt, ammonia buffer which undergo precipitation at 2 N sodium hydroxide solution at 80 °C for 1 h. The precipitates undergo repeated centrifugation at 4000 rpm for 5 min to remove the impurities. To eliminate residual moisture, they were initially dried in a hot air oven at 100 °C and subsequently calcined in a muffle furnace at 500 °C for 3 h. XRD, HRTEM, FESEM-EDS, and FTIR are used to analyze synthesized copper oxide nanoparticles, which have applications in inhibiting the bacterial growth. The synthesized particles exist as nanospheres with an average diameter of 50 nm and possess a monoclinic crystal structure. The anti-bacterial activities of copper oxide nanoparticles were evaluated by varying their concentrations from 25 mg/mL to 100 mg/mL. The results showed significant inhibition against Klebsiella pneumoniae with inhibition zone of 25 mm at 100 mg/mL CuO concentration, using the disc diffusion method.