We report micro-Raman spectroscopic studies of FeS2 pyrite in the diamond-anvil cell under hydrostatic and non-hydrostatic conditions to 55 GPa at room temperature. Four out of five Raman-active modes are resolved with helium as a pressure-transmitting medium to highest pressures. The fifth mode, Tg(2) [377 cm−1], is weak and unresolved lying ~2 cm−1 from the intense Ag mode [379 cm−1] at 1 bar. We observe an increase in the separation of the Eg [344 cm−1] and Tg(1) [350 cm−1] modes under compression. All observed frequencies increase continuously with increasing pressure showing no evidence for a structural phase transition in accord with both X-ray diffraction and shock-wave studies. The Ag and Tg(1) modes gain significantly in intensity relative to the Eg mode with increasing pressure probably resulting from Raman resonance effects. The Tg(3) mode [430 cm−1] broadens unusually compared to the other pyrite modes with pressure. The Raman data are consistent with a contraction of the S–S and Fe–S bonds under pressure. The main effect of non-hydrostatic conditions on the Raman modes is a strong pressure-induced broadening; the pressure-dependence of the frequencies and relative intensities are not affected within the error of the measurements.