The high-pressure structures and superconductivity of iodine-doped hydrogen have been studied by ab initio calculations. Above 100 GPa, we discover a stable phase with Pnma symmetry in the H₂I stoichiometry that consists of a monatomic iodine tube trapping hydrogen molecular units. Interestingly, H₂ molecular units dissociate and form a novel atomic phase with Rm symmetry at 246 GPa. Further electron–phonon coupling calculations predict the critical temperature of superconductivity T_c to be 3.8 K for the Pnma phase and 33 K for the Rm phase at 240 GPa. Significantly, the T_c of the Rm phase is enhanced approximately 8 times that of the Pnma phase, which is mainly attributed to the reason that H₂ molecules are broken exhibiting an atomic character in the Rm phase.