The development of nonfullerene acceptor materials applicable to organic photovoltaics (OPVs) has attracted considerable attention for the achievement of a high power conversion efficiency (PCE) in recent years. However, it is still challenging due to the insufficiency of both the variety of effective electron-deficient units and certain guidelines for the design of such materials. This work focusses on naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NTz) as a key electron-deficient unit. Therefore, a new electron-accepting π-conjugated compound (NTz-Np), whose structure is based on the combination of NTz and the fluorene-containing imide-annelated terminal units (Np), is designed and synthesized. The NTz-Np compound exhibits a narrow optical energy gap (1.73 eV), a proper energy level (−3.60 eV) of the lowest unoccupied molecular orbital, and moderate electron mobility (1.6 × 10−5 cm2 V−1 s−1), indicating that NTz-Np has appropriate characteristics as an acceptor against poly(3-hexylthiophene) (P3HT), a representative donor. OPV devices based on NTz-Np under the blend with P3HT show high photovoltaic performance with a PCE of 2.81%, which is the highest class among the P3HT/nonfullerene-based OPVs with the conventional device structure. This result indicates that NTz unit can be categorized as a potential electron-deficient unit for the nonfullerene acceptors.