Rugby-ball-shaped CoS₂ (rCoS₂) submicroparticles have been synthesized via a simple hydrothermal reaction. The rCoS₂ submicroparticles were then casted on a glassy carbon electrode and served as a highly conductive and large surface area platform for the electrosynthesis of poly(2-thiophenesulfonyl chloride) (pTSC), using 2-thiophenesulfonyl chloride as the monomer. The stepwise modification process was characterized by attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) and different electrochemical technologies. The stable pTSC layer was utilized as a functional interface to immobilize amino-modified probe DNA via a facile sulfonyl–amino condensation reaction, without the assistance of any other cross-linkers. Because of the synergistic effect of rCoS₂ and pTSC from the hybrid material, the developed DNA biosensor could detect target DNA over a wide dynamic range, from 1.0 × 10^–13 to 1.0 × 10^–7 M, with the detection limit down to 1.1 × 10^–14 M. Benefiting from the steady electropolymerization film of pTSC and the stable sulfamide bond between the amino-modified DNA and pTSC, the biosensor also showed high stability, reusability, and reproducibility.