  Using proton-proton collision data corresponding to an integrated luminosity
of 140 fb$^{-1}$ collected by the CMS experiment at $\sqrt{s}$ = 13 TeV, the
$\Lambda_\text{b}^0$ $\to$ J/$\psi\Xi^-$K$^+$ decay is observed for the first
time, with a statistical significance exceeding 5 standard deviations. The
relative branching fraction, with respect to the $\Lambda_\text{b}^0$ $\to$
$\psi$(2S)$\Lambda$ decay, is measured to be $\mathcal{B}$($\Lambda_\text{b}^0$
$\to$ J/$\psi\Xi^-$K$^+$)/$\mathcal{B}$( $\Lambda_\text{b}^0$ $\to$
$\psi$(2S)$\Lambda$) = [3.38 $\pm$ 1.02 $\pm$ 0.61 $\pm$ 0.03]%, where the
first uncertainty is statistical, the second is systematic, and the third is
related to the uncertainties in $\mathcal{B}$($\psi$(2S) $\to$
J/$\psi\pi^+\pi^-$) and $\mathcal{B}$($\Xi^-$ $\to$ $\Lambda\pi^-$).
