We present systematic ab initio calculations for the magnetic moments of 3d, 4d, and 5d transition-metal impurities as single adatoms on the (001) surfaces of Pd and Pt. Large magnetic moments are found for all 3d adatoms except for Sc and Ti, which are nonmagnetic. Among the 4d and 5d adatoms Mo, Tc, Ru and W, Re, Os have large magnetic moments. Also the adatom-induced magnetization of the Pd and Pt surfaces is investigated. We compare our results with former calculations for adsorbate atoms on Cu and Ag surfaces and for monolayers on these substrates.