The efficient separation of trace sulfur dioxide (SO2 ) is challenging and industrially important, but limited number of adsorbents have been reported with both desired selectivity and high capacity at the low concentration of SO2 . Here we demonstrate the deep removal of SO2 with high uptake capacity (1.55 mmol g‐1 ) and record SO2 /CO2 selectivity (>5000) at ultra‐low pressure of 0.002 bar, using ionic ultramicroporous polymers (IUPs) with high density of basic anions. The successful construction of uniform ultramicropores via polymerizing ionic monomers into IUPs enables the fully exploitation of the selective anionic sites. Notably, the aperture size and surface chemistry of IUPs can be finely tuned by adjusting the branched structure of ionic monomers, which play critical roles in excluding CH 4 and N 2 , as well as reducing the coadsorption of CO2 . Further, the swelling property of IUPs with adsorption of SO2 contributed to the high SO2 uptake capacity and high separation selectivity. Systematic investigations including static gas adsorption, dynamic breakthrough experiments, stability tests and modeling studies confirmed the efficient performance of IUPs for trace SO2 capture.