Combining metastable precursors with high pressure–temperature treatment is a powerful tool to make nitrogen-rich metal nitrides. Two new dense polymorphs of Ta3N5 have previously been theoretically predicted, with U3Se5 (Pnma) and U3Te5 (Pnma) structure types, and are now shown to exist. Amorphous Ta3N5 from thermal ammonolysis of an amorphous polymeric precursor was laser heated at 22 GPa and examined using synchrotron X-ray diffraction to reveal the emergence of these two novel polymorphs.