We have synthesized regioregular poly(3-alkylthiophene) (PST-HT) with different lengths of alkyl chains (CnH2n+1; n = 4, 8, 14, 20) attached to the polymer backbones. The chain-length-dependent thermochromic behavior was investigated by measurements of visible exciton absorption, photoluminescence, and IR vibrational absorption spectra. Optical spectra in ordered P3T-HT thin films show the nearly identical one-dimensional exciton state irrespective of the alkyl chain length, while the hysteretic thermochromic transition temperature decreases with the increase in the length. Upon the thermochromic phase transition, the order-disorder conformational change of alkyl chains occurs in a manner correlated with the conjugation length change of polythiophene backbone structures.