  The observation of the $\Lambda_\mathrm{b}^0$ $\to$ J/$\psi \Lambda \phi$
decay is reported using proton-proton collision data collected at $\sqrt{s} =$
13 TeV by the CMS experiment at the LHC in 2018, corresponding to an integrated
luminosity of 60 fb$^{-1}$. The ratio of the branching fractions
$\mathcal{B}(\Lambda_\mathrm{b}^0$ $\to$ J/$\psi \Lambda \phi)$ $/$
$\mathcal{B}(\Lambda_\mathrm{b}^0$ $\to$ $\psi \Lambda)$ is measured to be
(8.26 $\pm$ 0.90 (stat) $\pm$ 0.68 (syst) $\pm$ 0.11 $(\mathcal{B}))$ $\times$
10$^{-2}$, where the first uncertainty is statistical, the second is
systematic, and the last uncertainty reflects the uncertainties in the
world-average branching fractions of $\phi$ and $\psi$(2S) decays to the
reconstructed final states.
