<p>Molecular doping has emerged as a powerful strategy to tune the charge transport and mechanical properties of polymer semiconductors. However, the lack of structural diversity among dopants often limits the potential to improve both mobility and stretchability. Herein, we report a rationally designed Lewis acid dopant, i.e., branched octyloxy borane (BOB), which incorporates π-conjugated segments and branched alkoxy side chains to modulate polymer aggregation and crystallinity. When applied to a brittle diketopyrrolopyrrole-based polymer, BOB induces moderate p-type doping while suppressing long-range crystallinity in the solid state. At 1 wt% doping, the films show a 5-fold increase in crack onset strain (from 20 to 100%) and maintain a high mobility of 1.02 cm² V^–1 s^–1. The films retain more than 30% of their initial mobility at 100% strain and show excellent stability under repeated mechanical deformation. These findings provide insights into dopant‒polymer interactions and offer molecular design principles for dopants aimed at increasing the stretchability of polymer semiconductors.</p>