  We theoretically investigate Rydberg atom pair potentials of Rb atoms in
front of a perfectly conducting plate. The pair potentials are perturbed by
both the Casimir--Polder potential acting on a single atom and the scattering
contribution to the interatomic interaction. In contrast to the pair potentials
in free space, at atom-surface distances $d_s \lesssim 4\,\mu$m, avoided
crossings appear. In the associated potential wells that are entirely due to
dispersion interactions with the surface, there exist vibrational bound states,
i.e. Rydberg macrodimers, with groundstate energies of up to $E = -68\,$MHz and
radial expectation values of the order of several $\mu$m.
