A regioregular D-1-A-D-2-A terpolymer PDTSTTBDT incorporating dithieno[3,2-b:2',3'-d]silole (DTS, D-1) and benzo[1,2-b:4,5-b]dithiophene (BDT, D-2) units with perfectly controlled thieno[3,4-b]thiophene (TT, A) orientation was synthesized for the first time. The thermal, optical, and electrochemical properties of the regioregular PDTSTTBDT were characterized and compared with the random PDTSTTBDT without structural regioregularity. The regioregular PDTSTTBDT showed ideal optical bandgap (1.45 eV), lower lying HOMO energy level, and higher degree of crystallinity compared to the random PDTSTTBDT. Moreover, it exhibited excellent solubility in nonhalogenated solvents as well as halogenated solvents. The inverted bulk-heterojunction polymer solar cells (PSCs) based on the regioregular PDTSTTBDT and o-xylene process solvent showed a power conversion efficiency as high as 6.14%, which is 500% higher than the random PDTSTTBDT-based PSCs. It was found that the remarkable enhancement of photovoltaic performance in regioregular PDTSTTBDT-based PSCs is mainly due to improved light absorption, effective polymer ordering, and high charge carrier mobility.