A significant increase in compressive yield strength of the Al0.3CoCrFeMnNi high-entropy alloy (HEA) from 979 MPa to 1759 MPa was observed upon the introduction of 3 vol.%Y2O3. The HEAs were processed using spark plasma sintering of mechanically alloyed powders. Transmission electron microscopy and atom probe tomography confirmed the presence of compositionally complex nano-dispersoids in theY(2)O(3)-added HEA. The significant increase in strength can be attributed to the nano-dispersoid strengthening coupled with grain refinement. Therefore, the in-situ formation of the compositionally complex nanosca le dispersoids during the alloy processing could be a novel approach to create entropy-stabilized oxide particles in strengthening of HEAs.