Hollow-structured g-C₃N₄ polymers with a high thermal stability up to 550 °C and an enhanced photocatalytic activity have been developed by post-annealing treatment, which effectively modifies the textural, crystal, and electronic properties of the g-C₃N₄ semiconductors without extra chemical assistance. This is a unique example of thermally and chemically stable conjugated polymers with hollow nanostructures and optoelectronic properties, promising the development of functional hollow g-C₃N₄ nanocomposites by chemical modifications like doping, surface grafting, and coupling with other inorganic/polymeric semiconductors with the aid of thermal treatment at high temperatures.