Regioregular polythiophenes have been widely used in organic electronic applications due to their solution processability with chemical modification through side chain engineering, as well as their microstructural organization and good hole transport properties. Here, we introduce alkylthio side chains, (poly[(3-alkylthio)thiophene]s; P3ATTs), with strong noncovalent sulfur molecular interactions, to main chain thienyl backbones. These P3ATTs were compared with alkyl-substituted polythiophene (poly(3-alkylthiophene); P3AT) variants such that the effects of straight (hexyl and decyl) and branched (2-ethylhexyl) side chains (with and without S atoms) on their thin-film morphologies and crystalline states could be investigated. P3ATTs with linear alkylthio side chains (P3HTT, hexylthio; P3DTT, decylthio) did not attain the expected higher organic field-effect transistor (OFET) mobilities with respect to P3HT (hexyl) and P3DT (decyl) mainly due to their lower regioregularity (76−78%), although P3ATTs exhibit an enhanced tendency for aggregation and compact molecular packing, as indicated by the red-shifting of the absorption spectra and the shortening of the π−π stacking distance, respectively. Moreover, the loss of regioregularity issue can be solved by introducing more soluble 2-ethylhexylthio branched side chains to form poly [3-(2-ethylhexylthio)thiophene] (P3EHTT), which provides enhanced crystallinity and efficient charge mobility (increased by up to a factor of 3) with respect to the poly(2-ethylhexylthiophene) (P3EHT) without S atoms in the side moieties. This study demonstrates that the presence of side chain alkylthio structural motifs with nonbonded interactions in polythiophene semiconductors has a beneficial impact on the molecular conformation, morphologies, structural packing, and charge transport in OFET devices.