AbstractDiverse design strategies for nonfullerene acceptors (NFAs) have aided the development of efficient indoor organic photovoltaics (OPVs) for Internet‐of‐things‐enabled gadgets. However, the side‐chain approach to Y‐series NFAs for fabricating indoor OPVs remains largely unexplored. In this study, the synthesis and photovoltaic characteristics of two side‐chain engineered NFAs with halogenated end‐groups (BTP‐PO‐4F and BTP‐PO‐4Cl) are investigated. NFAs with a Y6 core unit are modified with butyloctyl inner side chains and ethylhexyloxynaphthyl outer side chains. The extended conjugation moiety contributed to red‐shifted absorption, with the chlorination effect presenting a more pronounced influence than fluorination. Further, the outer moiety introduced a slight electron‐withdrawing effect on the Y6 core unit, resulting in energy‐level downshifts. An excellent power conversion efficiency (PCE) of 23.2% is achieved in fluorinated OPVs under a light‐emitting diode lamp (1000 lux). Moreover, integrating the Y6 NFA into the binary blend further enhanced the PCE, reaching 28.3% in PM6:Y6:halogenated NFA ternary OPVs. The panchromatic absorption, high crystallinity, and cascaded energy levels of the ternary blends contribute to achieving a sufficient spectral match and suppress trap‐assisted recombination, ultimately fostering a substantially enhanced fill factor and PCEs under indoor lighting.