New photocatalysts comprising the 2D/2D heterojunction of graphitic carbon nitride (gCN) and molybdenum disulfide (MoS2) semiconductors doped with nickel (Ni) or cobalt (Co), denoted as gCN/MoS2-M (M: Ni, Co), were fabricated for the photocatalytic hydrogen evolution reaction (HER) under visible light illumination. First, the binary gCN/MoS2 heterojunctions were fabricated by using an in-situ solvothermal method and then they were doped with Ni or Co via a chemical reduction method. The photocatalytic HER experiments revealed that the prepared gCN/MoS2-M (M: Ni, Co) photocatalysts showed enhanced HER activities and stabilities compared to pristine gCN and binary gCN/MoS2 heterojunctions. Total H2 productions of 5924 μmol gcat−1 and 5159 μmol gcat−1 in 8 hours were provided by using gCN/MoS2-Ni and gCN/MoS2-Co photocatalysts, respectively, under visible light illumination. The detailed structural characterization and examination of optical properties of gCN/MoS2-M (M: Ni, Co) photocatalysts revealed that their enhanced photocatalytic activities were attributed to the formation of ‘type-I' 2D/2D heterojunction between gCN and MoS2 semiconductors and the creation of S-deficient MoS2 nanostructures after Ni or Co doping, which promoted the separation of the photogenerated electron-hole pairs, the charge mobility, and the visible light absorption.