The development of durable, low-cost, and efficient photo-/electrolysis for the oxygen and hydrogen evolution reactions (OER and HER) is important to fulfill increasing energy requirements. Herein, highly efficient and active photo-/electrochemical catalysts, that is, CoMn-LDH@g-C3N4 hybrids, have been synthesized successfully through a facile in situ co-precipitation method at room temperature. The CoMn-LDH@g-C3N4 composite exhibits an obvious OER electrocatalytic performance with a current density of 40 mAcm-2 at an overpotential of 350 mV for water oxidation, which is 2.5 times higher than pure CoMn-LDH nanosheets. For HER, CoMn-LDH@g-C3N4 (η50=-448 mV) requires a potential close to Pt/C (η50=-416 mV) to reach a current density of 50 mAcm-2. Furthermore, under visible-light irradiation, the photocurrent density of the CoMn-LDH@g-C3N4 composite is 0.227 mAcm-2, which is 2.1 and 3.8 time higher than pristine CoMn-LDH (0.108 mAcm-2) and g-C3N4 (0.061 mAcm-2), respectively. The CoMn-LDH@g-C3N4 composite delivers a current density of 10 mAcm-2 at 1.56 V and 100 mAcm-2 at 1.82 V for the overall water-splitting reaction. Therefore, this work establishes the first example of pure CoMn-LDH and CoMn-LDH@g-C3N4 hybrids as electrochemical and photoelectrochemical water-splitting systems for both OER and HER, which may open a pathway to develop and explore other LDH and g-C3N4 nanosheets as efficient catalysts for renewable energy applications.