In this work, Si_xCoCrNi (x = 0, 0.1, 0.2, 0.3, 0.4, molar ratio) alloys were designed by introducing elemental Si into CoCrNi, a medium-entropy alloy (MEA). The effects of Si addition on the microstructure and mechanical properties of the as-cast SiCoCrNi alloys were investigated. The results suggested that a novel Si-rich face-centred cubic (FCC) phase formed in the Si_0.3CoCrNi and Si_0.4CoCrNi alloys. The Si-rich FCC phase, which featured high hardness and was located between the dendrites of the matrix FCC phase (with higher phase fraction), significantly increased the yield strength of the Si_0.4CoCrNi alloys five times compared with the CoCrNi alloy.