Efficient electrocatalyst is essential to develop water splitting technology for large-scale hydrogen production, especially using bifunctional earth-abundant alternative catalysts under alkaline and neutral conditions. NiFe-based oxides have promising activity towards the oxygen evolution (OER). However, they usually show unsatisfactory performance for the hydrogen evolution reaction (HER), due to their large particle size with limited active sites and poor conductivity. Herein, we report the synthesis of sulfur-incorporated NiFe2O4 nanosheets on nickel foam (S-NiFe2O4/NF), composed of ultra-small nanoparticles (~ 2nm), by a facile confined growth strategy with the assistance of thiourea. Due to the favorable 3D hierarchical structure, the self-supported electrocatalyst endows abundant active sites, high electrical conductivity and rapid mass transfer, thereby achieving remarkable catalytic performance for overall water splitting under alkaline and neutral conditions. Specifically, the optimal S-NiFe2O4/NF electrode requires only 1.65 and 1.95V to deliver a current density of 10mAcm−2 in 1.0M KOH and 1.0M PBS, respectively, outperforming that of the commercial Pt/C as well as its counterparts (i.e. S-NiO/NF, S-Fe3O4/NF and pure NiFe2O4/NF). In addition, the synthesis strategy developed here can be applied to other mixed transition metal oxides with similar morphology and structure for various applications, such as supercapacitors, metal-air batteries, and photocatalysis.