  A discovery of the rare decay $B^{+} \rightarrow \pi^{+} \mu^{+} \mu^{-}$ is
presented. This decay is observed for the first time, with 5.2 $\sigma$
significance. The observation is made using $pp$ collision data, corresponding
to an integrated luminosity of 1.0 fb$^{-1}$, collected with the LHCb detector.
The measured branching fraction is (2.3 $\pm$ 0.6 (stat.) $\pm$ 0.1
(syst.))$\times 10^{-8}$, and the ratio of the $B^{+} \rightarrow \pi^{+}
\mu^{+} \mu^{-}$ and $B^{+} \rightarrow \K^{+} \mu^{+} \mu^{-}$ branching
fractions is measured to be 0.053 $\pm$ 0.014 (stat.) $\pm$ 0.001 (syst.).
