The electrochemical CO2 reduction reaction (CO2RR) offers a green and sustainable process to convert CO2 into valuable chemical stocks and fuels. Metal is one of the most promising types of catalysts to drive an efficient and selective CO2RR. The catalytic performance of metal nanocatalysts is strongly dependent on their structural features. Recently, phase engineering of nanomaterials (PEN) has emerged as a prominent tactic to regulate the catalytic performance of metal nanocatalysts for the CO2RR. A broad range of metal nanocatalysts with conventional and unconventional crystal phases has been developed, and remarkable achievements have been made. This review summarizes the most recent developments in phase engineering of metal nanocatalysts for the electrochemical CO2RR. We first introduce the different crystal phases of metal nanocatalysts used in the CO2RR and then discuss various synthetic strategies for unconventional phases of metal nanocatalysts. After that, detailed discussions of metal nanocatalysts with conventional and unconventional phases, including amorphous phases, are presented. Finally, the challenges and perspectives in this emerging area are discussed. Display omitted •Phase engineering of nanomaterials (PEN) emerges as a promising tactic to regulate their electrocatalytic performances.•Recent development in phase engineering of metal nanocatalysts for electrochemical CO2 reduction reaction was summarized.•Challenges and perspectives towards phase engineering of metal nanocatalysts for electrochemical CO2 reduction were proposed.