The interaction between thallium atoms and the Au(111) surface is studied using the cluster metal surface model and the density functional method. An adsorbed thallium atom forms a strong chemical bond with surface gold atoms. The adsorption energy barely depends on the location of the thallium atom. The electron density is appreciably displaced from thallium to gold in the process of adsorption. Thallium exists on the surface in the cationic form. The analysis of the density of state (DOS) spectra demonstrates that the atomic orbitals of thallium participate in the formation of lower molecular orbitals in the thallium–gold system when the surface is slightly filled with thallium. When the surface is filled to a substantial degree, the contribution of thallium to the molecular orbitals with the least negative energy appreciably grows. The possible change in the electronic work function upon the surface modification of gold with the adsorbed thallium is estimated.