The charge transport ability of polymer acceptors (PAs) is crucial for achieving high power conversion efficiencies (PCEs) of all‐polymer solar cells (all‐PSCs). However, the electron mobilities (μes) of most PAs are inferior to those of their small molecule acceptor (SMA) counterparts. Herein, the authors design a new series of the polymerized SMA‐based PAs (Y5‐A‐B), where the donating moiety (A = selenophene (Se)/biselenophene (BiSe)) and the backbone regioregularity (B = In/Mix/Out) are 2D controlled, for enhancing both the μe and PCEs. Interestingly, the effects of regioisomers on the μe and all‐PSC performance are the opposite depending on the donating unit. For the Y5‐Se‐based PAs, the PCEs increase in order of Out (7.52%) < Mix (9.33%) < In (13.38%). In contrast, for the Y5‐BiSe‐based PAs, the PCEs decrease in order of Out (10.67%) > Mix (9.58%) > In (8.52%). These opposite trends in each series originate from the different planarity and intermolecular assembly of PAs depending on the regioregularity. Thus, the Y5‐Se‐In blend exhibits the highest μe and achieves the highest PCE (13.38%) among the all‐PSCs in this study. Therefore, the authors report the importance of simultaneous engineering of the backbone building unit and regioregularity to realize high‐mobility PA and highly efficient all‐PSCs. A series of small molecule acceptor‐polymerized acceptors (PAs) with 2D control of regioregularity and ii) donating units are developed to investigate a direct relationship between the electron mobility of PA and the performance of all‐polymer solar cells (all‐PSCs). The Y5‐Se‐In‐based all‐PSC showed the most well‐developed crystalline properties, the electron mobility and, thus, the best efficiency of 13.4%. The authors report, for the first time, the opposite effects of regioisomers on the chain conformation, electron mobility of PAs, and the all‐PSC performance depending on the backbone building unit.