We report on measurements of the cross section for photoionization of Rb atoms in the highly-excited 5D5/2 level at wavelengths ranging from 1064 nm to 532 nm (photoelectron energies of 0.175 eV to 1.34 eV). We efficiently populate the 5D5/2 level using coherent two-photon excitation of trapped atoms with pulses in the counterintuitive order. The absolute photoionization cross sections are then measured via the increased loss of atoms from the magneto-optical trap when it is illuminated by the photoionizing light. Our results are in excellent agreement with new calculations based on the valence-electron parametric potential method.