We have performed a detailed study of the helium/Si(111)-(1 x 1)H interaction, using a thorough set of experimental measurements combined with a first principles theoretical approach. An empirical potential is presented, which describes both diffraction from the repulsive part and selective adsorption resonances resulting from the helium-surface potential well. The 80 meV potential energy contour has a corrugation of 0.6 Angstrom along the [(1) over bar 10] direction, and the well depth is 7.5 meV. The study is completed by a calculation of the helium/Si(111)-(1 x 1)H potential, based on the surface electron density using effective medium models. Comparison with the empirical potential provides insight into the nature of the helium-semiconductor interaction.