KEK. The T2K off-axis near detector, ND280, is used to make the first differential cross section measurements of muon neutrino charged current single positive pion production on a water target at energies ${∼}0.8$ GeV. The differential measurements are presented as a function of muon and pion kinematics, in the restricted phase-space defined by $p_{π^+} > 200$ MeV/c, $p_{μ^-} > 200$ MeV/c, $\cos\theta_{π^+} > 0.3$ and $\cos\theta_{μ^-} > 0.3$. The total flux integrated $ν_μ$ charged current single positive pion production cross section on water in the restricted phase-space is measured to be $〈σ〉_ϕ=4.25±0.48~(\mathrm{stat})±1.56~(\mathrm{syst})\times10^{-40}~\mathrm{cm}^{2}/\mathrm{nucleon}$. Data points taken from attached tar file available at http://t2k-experiment.org/results/. ; Other ; Unfolded $ν_μ$ CC1$π^+$ differential cross section as a function of $E_ν^{\rm rec}$ ($μ$, $π$) in the reduced phase-space of $p_{π^+} > 200$ MeV/$c$, $p_μ > 200$ MeV/c, $\cos(\theta_{π^+}) > 0.3$ and $\cos(\theta_μ) > 0.3$.