AbstractGenerally, highly efficient organic solar cells require both a high open‐circuit voltage (VOC) and a high short‐circuit current density (JSC). Reducing the energy loss (Eloss) is an effective way to achieve a high VOC without compromising the photocurrent, which is ideal for enhancing the power conversion efficiencies (PCEs). Herein, a new chlorinated nonfullerene acceptor (ITC‐2Cl) with chlorinated thiophene‐fused end groups is developed. In comparison with the unchlorinated counterpart (ITCPTC), the introduction of Cl improves not only the electronic properties by redshifting the absorption spectra and deepening the lowest unoccupied molecular orbital energy levels, but also the molecular packing and thus thin‐film morphology. The PM6:ITC‐2Cl‐based device yields a significantly higher PCE (13.6%) with a lower Eloss (0.67 eV) than the ITCPTC‐based device (PCE of 12.3% with Eloss of 0.70 eV). More importantly, compared to the archetypal nonfullerene acceptors such as IT‐4F (PCE of 12.9% with Eloss of 0.73 eV) and IT‐4Cl (PCE of 12.7% with Eloss of 0.76 eV), the ITC‐2Cl‐based device shows a higher PCE and a lower Eloss. These results demonstrate that the chlorinated thiophene‐fused end group is a promising candidate for a high‐performance nonfullerene acceptors with low energy loss.