We found a new nickel sulfide that is isostructural with Fe3S. The synthesized nickel sulfide is a non-stoichiometric compound with a Ni deficiency and its composition is Ni2.90±0.5S. In situ synchrotron X-ray observations indicate that Ni3–xS forms above 5.1 GPa and melts incongruently into Ni+liquid up to 10 GPa. The bulk modulus of Ni3–xS at 300 K was determined to be 140 ± 2 GPa with a fixed K′ = 4 by static compression with a liquid pressure medium. The eutectic point of the Ni-NiS system lies between Ni3S and Ni3S2 up to 10 GPa and its composition changes from Ni66.6S33.4 at 0.1 MPa to Ni70.7S29.3 at 10 GPa. The eutectic melting temperature of the Ni-NiS system decreases to 5.1 GPa and 720 K as the pressure increases where Ni and α-Ni3S2 are the eutectic solids. On the other hand, the eutectic melting temperature increases with a positive slope above 5.1 GPa where Ni3S+α-Ni3S2 is stable under subsolidus conditions. Intermediate compounds appear at a lower pressure in the Ni-NiS system than that for the Fe-FeS system. Ni3S2 is stable at atmospheric pressure and Ni3S forms at 5.1 GPa, whereas Fe3S2 and Fe3S form at 14 and 20 GPa, respectively. The addition of Ni complicates the melting relationship in the Fe-FeS system at high pressure because of the wider stability field of Ni3S. The low-melting temperature of the Fe-Ni-S system plays an important role in the percolative core-formation of planetesimals during planetary accretion.