We present the density functional calculation results for K adsorption on Pt(111) in a (√3 × √3)R30° structure. The site preference, surface relaxation, work function, and electron structure of the system are analyzed. The hcp hollow position is found to be the most favorable for K adsorption. The calculated surface relaxations and adsorption geometry are in agreement with available experimental data. It is demonstrated that the K adsorption leads to the disappearance of a number of platinum surface and resonance states in the energy region above −2 eV and to the appearance of new platinum surface features, as well as bands that are significantly localized at the adsorbate. It is found that the K adsorption 1) transforms the Shockley surface state lying in the bulk band gap near the Γ¯ point on the clean Pt surface into the state localized at the K adlayer and 2) pushes this state up in energy by about 0.17 eV relative to the bottom of the bulk band gap. It is shown that the Rashba spin-orbit strength parameter in this state is surprisingly greater than the respective parameter in the Shockley surface state on Au(111) and Bi(111).