The Cabibbo-suppressed semileptonic decay B + → ppµ + ν µ is observed for the first time using a sample of pp collisions corresponding to an integrated luminosity of 1.0, 2.0 and 1.7 fb −1 at centre-of-mass energies of 7, 8 and 13 TeV, respectively. The differential branching fraction is measured as a function of the pp invariant mass using the decay mode B + → J/ψK + for normalisation. The total branching fraction is measured to be B(B + → ppµ + ν µ) = (5.27 +0.23 −0.24 ± 0.21 ± 0.15) × 10 −6 , where the first uncertainty is statistical, the second systematic and the third is from the uncertainty on the branching fraction of the normalisation channel.