A potential-energy surface for the water-helium complex is constructed from scaled perturbation theory calculations, and calibrated using accurate supermolecule methods. At the global minimum. the helium atom lies in the plane of the water molecule with an interaction energy corresponding to about 35 cm(-1) (-160 microhartree). The potential is used to calculate second virial coefficients. including first-order quantum corrections, from 100 to 2000 K. The estimated uncertainties in the calculated values are much smaller than the uncertainties in the available experimental data; the calculated values also cover a much wider range of temperature. The quantum corrections are found to be smaller in magnitude than the uncertainty in the calculated second virial coefficient.