Using a data sample corresponding to an integrated luminosity of $103.7~\mathrm{fb}^{-1}$ of pp collisions, collected by the CMS experiment at $\sqrt{s}=13~\mathrm{TeV}$ in $2017$$-$$2018$, the $\mathrm{B}^0\to\psi\mathrm{(2S)}\mathrm{K}^0_{\mathrm S}\pi^+\pi^-$ and $\mathrm{B}^0_{\rm s}\to\psi\mathrm{(2S)}\mathrm{K}^0_{\mathrm S}$ decays are observed for the first time. Measurements of their branching fractions, relative to the $\mathrm{B}^0\to\psi\mathrm{(2S)}\mathrm{K}^0_{\mathrm S}$ decay, are presented.