We report the first direct measurement of decays of the $Λ^+_c$ baryon involving the neutron. The analysis is performed using 567 pb$^{-1}$ of $e^+e^-$ collision data collected at $\sqrt{s}=4.599$ GeV with the BESIII detector at the BEPCII collider. We observe the decay $Λ^+_{c}→ n K^0_Sπ^+$ and measure the absolute branching fraction to be $\mathcal{B}({Λ^+_{c}→ n K^0_Sπ^+})=(1.82±0.23({\rm stat})±0.11({\rm syst}))\%$. A comparison to $\mathcal{B}({Λ^+_{c}→ p(\bar{K}π)^0})$ provides an important test of isospin symmetry and final state interactions.