The flavour-changing neutral currents of the rare decays $B^{0}_{(s)} \rightarrow \mu^{+}\mu^{-}$ provide a favourable environment to observe new physics. This contribution summarises the study of these rare decays, using the 2015 and 2016 data collected by the ATLAS detector. Their branching ratios are measured relative to the reference decay mode $B^{\pm} \rightarrow J/\psi K^{\pm}$, which has a well-measured branching fraction $\mathcal{B}(B^{\pm} \rightarrow J/\psi K^{\pm}) \times \mathcal{B}(J/\psi \rightarrow \mu^{+}\mu^{-})$. The event yields of the reference and the rare-decay channels are extracted employing the unbinned maximum likelihood fit approach. Run 2 (2015+2016) and Run 1 ATLAS measurements are combined. The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the $\mathcal{B}$($B^{0}_{s} \rightarrow \mu^{+}\mu^{-}$)-$\mathcal{B}$($B^{0} \rightarrow \mu^{+}\mu^{-}$) plane.