AbstractThe establishment of methodologies for encapsulating polymers in small molecular hosts is not only highly desirable to facilitate new material functions but also very important for the development of fundamental science. Herein a new strategy for encapsulating noncrystalline polymer chains with bulky side groups in thiourea (TU) crystals is disclosed. In the process, the noncrystalline polymer is first complexed with TU and 1,4‐dioxane molecules, forming a crystalline metastable complex structure with melting temperature below 110 °C. Subsequent removal of 1,4‐dioxane through heating the complex at 120 °C collapses the metastable complexes, but surprisingly affords a polymer‐TU inclusion compound (IC) structure, as demonstrated by infrared radiation, differential scanning calorimetry, and X‐ray powder diffraction. The strategy is applicable to both atactic‐poly (methyl methacrylate) and ‐polystyrene, suggesting that it might be a universal method for encapsulating noncrystalline polymer chains in thiourea crystal canals.