An in situ synchrotron powder X-ray diffraction study on (Fe0.89,Ni0.11)3S was conducted up to 141 GPa and 1590 K. (Fe0.89,Ni0.11)3S has a tetragonal structure, which is the same structure as Ni-free Fe3S. Fitting a third-order Birch-Murnaghan equation of state to data at ambient temperature yielded a bulk modulus of K0 = 138.1(7.2) GPa and its pressure derivative K0′ = 4.5(3) with a zero pressure volume V0 = 375.67(4) Å3. The density of (Fe0.89,Ni0.11)3S under the core-mantle boundary condition is 1.7% greater than that of Fe3S. The axial ratio (c/a) of (Fe0.89,Ni0.11)3S decreases with increasing pressure. The addition of nickel to Fe3S leads to a softening of the c-axis. Assuming that the nickel content of the outer core is about 5 at%, we estimated 12.3–20.8 at% sulfur in the outer core for the given 6–10% density deficit between the outer core and pure iron at 136 GPa.