  Based on 586 $\rm{pb^{-1}}$ of $e^+e^-$ annihilation data collected at a
center-of-mass energy of $\sqrt{s}=4.6~\rm{GeV}$ with the BESIII detector at
the BEPCII collider, the absolute branching fraction of $\Lambda_c^+ \to p
K^0_{\mathrm{S}}\eta$ decays is measured for the first time to be
$\mathcal{B}(\Lambda_c^+ \to p K^0_{\mathrm{S}}\eta) = (0.414 \pm 0.084 \pm
0.028)\%$, where the first uncertainty is statistical and the second is
systematic. The result is compatible with a previous CLEO result on the
relative branching fraction $\frac{\mathcal{B}(\Lambda_c^+ \to p
K^0_{\mathrm{S}}\eta)}{\mathcal{B}(\Lambda_c^+ \to p K^-\pi^+)}$, and
consistent with theoretical predictions of SU(3) flavor symmetry.
