  Measurements are presented of the B$^0_\mathrm{S}$ $\to$ $\mu^+\mu^-$
branching fraction and effective lifetime, as well as results of a search for
the B$^0$ $\to$ $\mu^+\mu^-$ decay in proton-proton collisions at $\sqrt{s}$ =
13 TeV at the LHC. The analysis is based on data collected with the CMS
detector in 2016-2018 corresponding to an integrated luminosity of 140
fb$^{-1}$. The branching fraction of the B$^0_\mathrm{S}$ $\to$ $\mu^+\mu^-$
decay and the effective B$^0_\mathrm{S}$ meson lifetime are the most precise
single measurements to date. No evidence for the B$^0$ $\to$ $\mu^+\mu^-$ decay
has been found. All results are found to be consistent with the standard model
predictions and previous measurements.
