We report the first measurement of the absolute branching fraction of the inclusive decay $$\Lambda _c^+ \rightarrow K_S^0X$$ ${\Lambda }_c^{+}\to K_S^{0}X$ . The analysis is performed using an $$e^+e^-$$ $e^{+}{e}^{-}$ collision data sample corresponding to an integrated luminosity of 567 $$\hbox {pb}^{-1}$$ ${\text{pb}}^{-1}$ taken at $$\sqrt{s}$$ $\sqrt{s}$ = 4.6 GeV with the BESIII detector. Using eleven Cabibbo-favored $${\bar{\Lambda }}_c^-$$ ${\overline{\Lambda }}_c^{-}$ decay modes and the double-tag technique, this absolute branching fraction is measured to be $${\mathcal {B}}(\Lambda _c^+ \rightarrow K_S^0X)=(9.9\pm 0.6\pm 0.4)\%$$ $B({\Lambda }_c^{+}\to K_S^{0}X)=(9.9\pm 0.6\pm 0.4)\%$ , where the first uncertainty is statistical and the second systematic. The relative deviation between the branching fractions for the inclusive decay and the observed exclusive decays is $$(18.7\pm 8.3)\%$$ $(18.7\pm 8.3)\%$ , which indicates that there may be some unobserved decay modes with a neutron or excited baryons in the final state.