The liquid crystalline (LC) characteristics of conjugated polymers (CPs) can result in their self-assembly with long-range ordering, providing an effective route for the formation of an optimized morphology suitable for high-performance organic electronic devices. In this study, the temperature-dependent LC phases of regioregularity (RR)-controlled poly(3-decylthiophene)s (P3DDTs) are systematically investigated. Controlling the RR of P3DDTs from 94 to 60% significantly modifies the strengths of their LC interactions, whereas their chemical structures remain almost the same. As the RR decreases, P3DDTs exhibit progressively decreased phase transition temperatures, i.e., the nematic-to-isotropic transition temperature, as measured using polarized optical microscopy (POM), UV-vis spectroscopy, differential scanning calorimetry (DSC), and X-ray scattering. P3DDTs with a high RR (94 and 83%) exhibit only the Form I crystal structures that are typically observed in most regioregular poly(3-alkylthiophene)s. However, P3DDTs with a low RR (76 and 60%) adopt both the Form I and Form II crystal structures, where Form II crystals contain interdigitated alkyl side-chain stackings. The amount of Form I crystals is significantly decreased as the RR decreases, and Form II crystals are dominantly observed for RR 60% P3DDT polymers. These features explain the significant decreases in the transition temperatures between the phases for P3DDTs with lower RR. This study provides an understanding of the RR effect in determining the crystalline structures and thermotropic LC behavior of P3DDTs, allowing for exquisite control over their self-assembly and optoelectrical properties.