The chemisorption of Cl atoms on the γ-TiAl(111) surface is investigated by density functional theory. The calculated results show that the more stable sites for Cl atoms adsorption are the surface face-centred cubic (fcc) sites and the surface hexagonal close-packed (hcp) sites. When the coverage of Cl is less than 1 monolayer (ML), Cl atoms prefer the adsorption site with more Ti atoms as its nearest neighbors on the surface layer. From the analysis of the electronic structures, it can be found that the bonds formed by Cl and metal atoms are mainly ionic and directional. When Cl and O atoms are co-adsorbed on γ-TiAl(111) surface, both of them prefer the fcc and hcp sites. As a result, there is a competition between them. Furthermore, the adsorption energy per oxygen atom is increased by the adsorption of Cl atoms, which indicates that interactions between oxgen and metal atoms are weakened by the adsorption of Cl atoms on γ-TiAl(111) surface. This may be one of the reasons why the oxidation resistance of γ-TiAl can be improved by chlorine treatment.