The application of pressure allows for systematic tuning of the charge density of a material "cleanly", i.e. without changes to the chemical composition via dopants, and exploratory high pressure experiments can inform the design of bulk syntheses of materials that benefit from their properties under compression. Here, we report the electronic and structural response of semiconducting tin nitride Sn3N4 under compression – a continuous opening of the optical band gap from 1.3 eV to 3.0 eV over a range of 100 GPa, a 540 nm blueshift spanning the entire visible spectrum. The pressure-mediated band gap opening is general to this material across numerous high-density polymorphs, implicating the predominant ionic bonding in the material as the root of its mechanism – fingerprinted by increased charge localisation with reduced volume. The rate of decompression to ambient conditions permits access to recoverable metastable states with varying band gaps energies, opening the possibility of pressure tuneable electronic properties for future applications.