The ATLAS experiment at the Large Hadron Collider has performed measurements of the rare flavor-changing neutral-current processes $b \to s \mu^+\mu^-$ and $B^0_{(s)} \to \mu^+\mu^-$which are sensitive to New Physics effects. This contribution presents recent ATLAS results from the angular analysis of the $B^0 \to K^{\star0} \mu^+\mu^-$ decay with LHC Run 1 data, the $BR(B^0_{(s)} \to \mu^+\mu^-)$ measurement with 2015 and 2016 data as well as projections for the $B^0 \to K^{\star0} \mu^+\mu^-$ and $BR(B^0_{(s)} \to \mu^+\mu^-)$ measurements for the High-Luminosity LHC phase.