Developing an efficient and economical electrocatalyst for oxygen evolution reaction is the key challenge to renewable energy technologies. Metal selenides are attractive candidates for electrocatalytic water oxidation because they provide suitable surface-active sites for the reaction. Herein, we report less explored hexagonal cobalt selenide (CoSe) preparation for oxygen evolution reaction through the facile and environmentally benign one-step hydrothermal method. Reaction conditions were precisely tailored to develop highly crystalline and porous dendrimeric architectures of CoSe. Owing to its exclusive porous and dendrimeric crystalline network and large electrochemical surface area, the superior CoSe electrocatalyst (that is 16H) showed excellent electrochemical activity with remarkably low overpotential (250 mV at 10 mA cm−2) and very high current density (570 mA cm−2) in a small potential window. The Tafel slope of 16H sample was 56 mV dec−1, indicating the faster kinetics at the catalyst surfaces. Moreover, it also showed excellent stability under harsh oxidative conditions in a 24-hour long stability test experiment.