  A search for the decay $B_{s}^0 \rightarrow \overline{K}{}^{*0}\mu^+\mu^-$ is
presented using data sets corresponding to 1.0, 2.0 and 1.6 $\text{fb}^{-1}$ of
integrated luminosity collected during $pp$ collisions with the LHCb experiment
at centre-of-mass energies of 7, 8 and 13 TeV, respectively. An excess is found
over the background-only hypothesis with a significance of 3.4 standard
deviations. The branching fraction of the $B_{s}^0 \rightarrow
\overline{K}{}^{*0}\mu^+\mu^-$ decay is determined to be $\mathcal{B}(B_{s}^0
\rightarrow \overline{K}{}^{*0}\mu^+\mu^-) = [2.9 \pm 1.0~(\text{stat}) \pm
0.2~(\text{syst}) \pm 0.3~(\text{norm})] \times 10^{-8}$, where the first and
second uncertainties are statistical and systematic, respectively. The third
uncertainty is due to limited knowledge of external parameters used to
normalise the branching fraction measurement.
