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 the muon and pion kinematics, in the restricted phase space defined by
p
π
+
>
200
 
 
MeV
/
c
,
p
μ
>
200
 
 
MeV
/
c
,
cos
(
θ
π
+
)
>
0.3
and
cos
(
θ
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
(
stat
)
±
1.56
(
syst
)
×
10
−
40
 
 
cm
2
/
nucleon
. The total cross section is consistent with the NEUT prediction (
5.03
×
10
−
40
 
 
cm
2
/
nucleon
) and
2
σ
lower than the GENIE prediction (
7.68
×
10
−
40
 
 
cm
2
/
nucleon
). The differential cross sections are in good agreement with the NEUT generator. The GENIE simulation reproduces well the shapes of the distributions, but overestimates the overall cross section normalization.