The photoproduction of prompt photons, together with an accompanying jet, has been measured with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb−1. A study of the effective transverse momentum, 〈kT〉, of partons in the proton, as modelled within the framework of the PYTHIA Monte Carlo, gives a value of 〈kT〉=1.69±0.18+0.18−0.20 GeV for the γp centre-of-mass energy range 134<W<251 GeV. This result is in agreement with the previously observed trend in hadron–hadron scattering for 〈kT〉 to rise with interaction energy.