Lattice strains in nanocrystalline cubic silicon nitride were measured using an energy-dispersive x-ray diffraction technique under nonhydrostatic stress conditions up to a confining pressure of . The high-pressure elastic properties of were also investigated theoretically using density-functional theory. The differential stress between 30 and increases from and can be described beyond as where is the pressure in GPa. The differential stress supported by increases with pressure from 3.5% of the shear modulus at to 7.6% at . is one of the strongest materials yet studied under extreme compression conditions. The elastic anisotropy of is large and only weakly pressure dependent. The elastic anisotropy increases from to as the parameter that characterizes stress-strain continuity across grain boundaries is decreased from 1 to 0.5. The high elastic anisotropy compares well with our first-principles calculations that lead to at ambient pressure and at . Using molybdenum as an internal pressure standard, the equation of state depends strongly on , the direction between the diamond cell axis and the normal of the scattering plane. The bulk modulus increases from as varies from 0° to 90°. This large variation highlights the need to account properly for deviatoric stresses in nonhydrostatic x-ray diffraction experiments carried out at angles other than the particular angle of , where deviatoric stress effects on the lattice vanish. At this angle we find a bulk modulus of (, fixed). This result is in general agreement with our local density approximation calculations, , , and previous shockwave and x-ray diffraction studies. However, our results are significantly lower than the recently reported bulk modulus of for nanocrystalline below .