Developing high-efficiency and earth-abundant electrocatalysts for electrochemical water splitting is of paramount importance for energy conversion. Although tremendous effort has been paid to transition metal (TM) material-based electrocatalysts, rational design and controllable synthesis of fine structures to fully utilize the latent potential of TM materials remain great challenges. We herein report a composition-tuning strategy to achieve rational structure control of quaternary Co-Ni-S-Se materials through a facile one-pot hydrothermal method, in which earth-abundant Ni is introduced into a CoSxSe2-x matrix to optimize the morphology and electronic structure of the quaternary electrocatalyst. Because of the introduction of Ni, this novel Co-Ni-S-Se quaternary system shows better catalytic activity for water splitting with Tafel slopes of 42.1 mV dec(-1) for hydrogen evolution reaction (HER) and 65.5 mV dec(-1) for oxygen evolution reaction (OER), respectively, compared with its precursor Co-S-Se ternary system. For stability, there is negligible fading after long-term electrochemical test. Our work not only provides a novel thinking to introduce nickel into Co-S-Se ternary system by a facile hydrothermal synthesis for electrochemical water splitting, but also this quaternary system realizes bifunctional catalysis and better electrochemical performance relative to the ternary counterpart. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.