Over the past two decades, the charge carrier mobility of π-conjugated polymers has vastly improved. This has been mostly achieved by increasing the π–π stacking ability of the polymers through advanced molecular design, thereby improving “interchain” charge carrier transport. However, the rational design of π-conjugated polymers for improving “intrachain” charge carrier transport along the backbone still remains a formidable challenge. Here, we show the synthesis of a new π-conjugated polymer based on a quinoidal bithiophene moiety (PSP4T), which interestingly, was found to have significantly extended π-electron delocalization along the backbone compared to its isomer (PBTD4T), although these polymers have an identical basic structure. Importantly, despite the similar π–π stacking structure, PSP4T demonstrated transistor mobilities of around 1–2.5 cm2 V–1 s–1 that are 1–2 orders of magnitude higher than that of PBTD4T. On the basis of further investigations of energetic disorder and theoretical simulations, the higher mobility in PSP4T than in PBTD4T is most likely attributed to the remarkably higher intrachain charge carrier transport, which originates in the highly extended π-electron delocalization. We believe that our study can provide new guidelines for the design of π-conjugated polymers with high intrachain charge carrier transport.