A new s-tetrazine-based low-bandgap semiconducting polymer, PCPDTTTz, was designed and synthesized. This is the first solution-processable conjugated polymer with tetrazine in the main chain. This polymer shows good thermal stability and broad absorption covering 450-700 nm. The HOMO and LUMO energy levels were estimated to be -5.34 and -3.48 eV, with an electrochemical bandgap of 1.86 eV. Simple polymer solar cells based on PCPDTTTz and PC71BM exhibit a calibrated power conversion efficiency of 5.4%.