A discovery of the rare decay B + → π + μ + μ − is presented. This decay is observed for the first time, with 5.2 σ 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 ± 0.6 (stat.) ± 0.1 (syst.))×10−8, and the ratio of the B + → π + μ + μ − and B + → K + μ + μ − branching fractions is measured to be 0.053 ± 0.014 (stat.) ± 0.001 (syst.).