The electrochemical CO₂ reduction reaction (CO₂RR) has become a promising technology to resolve globally accelerating CO₂ emissions and produce chemical fuels. In this work, the electrocatalytic performance of transition metal (TM = Cu, Cr, Mn, Co, Ni, Mo, Pt, Rh, Ru and V) triatomic clusters embedded in a graphdiyne (GDY) monolayer (TM₃@GDY) for CO₂RR is investigated by density functional theory (DFT) calculations. The results indicate that Cr₃@GDY possesses the best catalytic performance with a remarkably low rate-limiting step of 0.39 eV toward the CO₂ product, and it can also effectively suppress the hydrogen evolution reaction (HER) during the entire CO₂RR process. Studies on the rate-limiting steps (CHO* + H⁺ + e⁻ → CHOH) of Cr_n@GDY (n = 1–4) structures demonstrate that the high catalytic performance is attributed to the strong synergistic reaction of three Cr atoms interacting with the C atom for the Cr₃@GDY structure. The strong synergistic reaction gives rise to the weakest interaction between O–Cr atoms, which leads to the strongest interaction between O–H atoms and makes the hydrogenation process easier for the Cr₃@GDY structure. Furthermore, ab initio molecular dynamics simulations (AIMD) at 500 K reveal the high thermodynamic stability of the Cr₃@GDY structure. These studies may provide a new approach for designing highly efficient electrocatalysts for the CO₂RR under ambient conditions.