In this work, we develop mechanically robust and high-performance organic thin-film transistors (OTFTs) based on poly(3-hexylthiophene) (P3HT) regioblock copolymers (block-P3HTs). These block-P3HTs consist of regioregular (rre) and regiorandom (rra) P3HTs, where the highly crystalline rre block allows efficient charge transport while the amorphous rra block provides mechanical robustness and interdomain connection. To examine the effects of the molecular architecture on the OTFT performance and stretchability, we prepare a series of block-P3HTs having different number-average molecular weight (M-n) values of rra blocks (from 0 to 32 kg mol(-1)) and a fixed M-n of rre blocks (11 kg mol(-1)). Thin films of all of the block-P3HTs exhibit a high charge-carrier mobility due to the formation of well-developed edge-on crystallites from the rre blocks confined within the rra domains, leading to a hole mobility of 1.5 X 10(-1) cm(2) V-1 s(-1), which is superior to that of the rre P3HT homopolymer. In addition, the mechanical toughness of block-P3HT thin films is remarkably enhanced by the rra block. While the rre P3HT homopolymer thin film shows a brittle behavior with an elongation at break of only 0.3%, the elongation at break of the block-P3HT thin films increases by a factor of 100, yielding 30.2% with increasing M-n of the rra block, without sacrificing the electrical properties. In particular, a noticeable enhancement of both elongation at break and toughness is observed between M-n values of the rra block of 8 and 20 kg mol(-1), indicating that the critical molecular weight of rra P3HT plays an important role in determining the mechanical response of the block-P3HT thin films. This study provides guidelines and strategies to improve the mechanical properties of organic electroactive materials without the disruption of optoelectrical properties, which is critical to fabricate high-performance soft electronics.