Small molecule acceptors with wide absorption spectra and small bandgap are very important for the sunlight harvesting of organic solar cells (OSCs). Recently, we have developed A–D–A′–D–A type small molecule acceptors with wide absorption spectra. However, the molecule (IID-IC) suffers from medium bandgap with poor light absorption in the near-infrared (NIR) region. Here, we develop a new A–D–A′–D–A type small molecule acceptor, IID-IC-O, with both wide absorption spectrum and small bandgap. IID-IC-O is designed by replacing the alkyl substituents of IID-IC with alkoxyl substituents. The replacement leads to upshifted HOMO energy level and improved backbone planarity of IID-IC-O. As a result, IID-IC-O exhibits a wide absorption spectrum spanning from 300 nm to 890 nm with a bandgap as small as 1.39 eV. The OSC device of IID-IC-O exhibits a power conversion efficiency of 4.20% with wide photoresponse from 300 nm to 900 nm. These results demonstrate the superior light harvesting capability of rationally designed organic molecules, which may be useful for many solar energy-related applications.