The appropriate band structure endows graphitic carbon nitride (g-C₃N₄) with benign redox ability and visible light response, resulting in its popularity in photocatalysis. Given the inferior solar-to-chemical (STC) energy conversion of single-component g-C₃N₄, loading cocatalysts is serviceable in advancing its photocatalytic activity. In particular, two-dimensional (2D) cocatalysts that could form 2D/2D heterojunctions with g-C₃N₄ stand out due to several advantages in which the large-area contact interface with g-C₃N₄ predominates. Herein, the basic information of g-C₃N₄ was first introduced. Then, representative 2D cocatalysts (e.g., graphene, graphdiyne, molybdenum disulfide, black phosphorus, and MXenes) used to strengthen the STC energy conversion of g-C₃N₄ were presented. Afterwards, the foremost achievements of g-C₃N₄ decorated with 2D cocatalysts in STC energy conversion were described in terms of photocatalytic hydrogen evolution, carbon dioxide reduction, hydrogen peroxide production, and nitrogen fixation. Finally, the future development and challenge of photocatalysts decorated with 2D cocatalysts were prospected. This paper could hopefully deepen the readers’ understanding of 2D cocatalysts in photocatalysis and attach importance to 2D cocatalysts described in this paper and many others not mentioned. Copyright © 2021 Elsevier Ltd. All rights reserved.