The first Brillouin‐scattering measurements of single‐crystal Mg2SiO4have been made at 1 atm and pressures up to 40 kbar in a diamond‐anvil cell at room temperature. The elastic moduli C11,C22,C33,C44,C55, and C66, and their pressure derivatives were obtained. These results were compared with those obtained by ultrasonic measurements up to 10 kbar. The observation of lowdCij/dPslopes for the transverse moduli. C44, C55, and C66, and the evidence for negative second derivatives as well, support Poirier’s proposed mechanism for the olivine→spinel transition due to gliding of close‐packed oxygen layers. Moreover, the numerical relationships among photoelastic constants were determined by studying the intensity ratio of LA to TA modes.