Lithium–sulfur (Li–S) batteries offer very high theoretical capacity and energy density, but their practical application is limited by poor sulfur conductivity and the polysulfide shuttle effect, which leads to rapid capacity fading. In this work, polymer of intrinsic microporosity (PIM-1) was investigated as a sulfur host to improve cathode stability. PIM-1 possesses intrinsic micropores (90% capacity retention and nearly 100% coulombic efficiency. The improved performance is attributed to the microporous confinement of sulfur and adsorption of lithium polysulfides by the polar nitrile (–CN) groups of PIM-1, which suppresses the shuttle effect. These results highlight the potential of microporous PIM-1 as an effective polymer host for stabilizing sulfur cathodes in Li–S batteries.