The novel high-temperature polymorph of sodium imidazolate reported in this paper was discovered in the mechanochemical reaction of NaIm with NaBH4 and later prepared directly by thermal treatment of the room-temperature polymorph of NaIm. Solid-state NMR was used for initial elucidation of structural features; the crystal structure was determined by single-crystal X-ray diffraction, while the in situ HT-XRPD experiments utilizing synchrotron radiation have been performed in order to gain the insight into the structural evolution and thermal stability, in addition to differential thermal analysis and hot-stage microscopy measurements. Contrary to the RT polymorph that forms a dense and hypercoordinated structure without pores, the high-temperature polymorph of NaIm exhibits pores of 50 Å3 that suggest possible application for gas sorption/separation. It is important to highlight that, once formed, the high-temperature polymorph of NaIm retains its structure and remains stable at room temperature.