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 pπ+>200MeV/c, pμ− >200MeV/c, cosθπ+ >0.3 and cosθμ− >0.3. The total flux integrated νμ 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 (stat)±1.56 (syst)×10−40 cm2/nucleon. The total cross section is consistent with the NEUT prediction (5.03×10−40 cm2/nucleon) and 2σ lower than the GENIE prediction (7.68×10−40 cm2/nucleon). The differential cross sections are in good agreement with the NEUT generator. The GENIE simulation reproduces well the shapes of the distributions, but over-estimates the overall cross section normalization.