The structure of realgar, As4S4, and its evolution with pressure have been investigated employing in situ X-ray diffraction, optical absorption and vibrational spectroscopy on single-crystal samples in diamond-anvil cells. Compression under true hydrostatic conditions up to 5.40 GPa reveals equation-of-state parameters of V 0 = 799.4(2.4) Å3 and K 0 = 10.5(0.4) GPa with K_0^' = 8.7. The remarkably high compressibility can be attributed to a denser packing of the As4S4 molecules with shortening of the intermolecular bonds of up to 12 %, while the As4S4 molecules remain intact showing rigid-unit behaviour. From ambient pressure to 4.5 GPa, Raman spectra exhibit a strong blue shift of the Raman bands of the lattice-phonon regime of 24 cm-1, whereas frequencies from intramolecular As-S stretching modes show negligible or no shifts at all. On pressurisation, realgar shows a continuous and reversible colour change from bright orange over deep red to black. Optical absorption spectroscopy shows a shift of the absorption edge from 2.30 to 1.81 eV up to 4.5 GPa, and DFT calculations show a corresponding reduction in the band gap. Synchrotron-based measurements on polycrystalline samples up to 45.5 GPa are indexed according to the monoclinic structure of realgar.