In this paper, we consider a one-loop thermal correction to the g-factors of the ground and low-lying excited states in atomic systems with one valence s electron. As an example, we carried out calculations for Rb and Cs atoms, where optical measurements of transition frequencies set the metrological level of accuracy. With the development of atomic systems as the most accurate tool for spectroscopic experiments, it has become increasingly important to improve the accuracy of measuring the electron g-factor, which is currently at10−12. The rapid progress in this field in recent years requires analysis not only of various relativistic, quantum electrodynamics, and other effects, but also theoretical studies of phenomena stimulated by the external thermal environment. We can expect that the thermal contribution to the g-factor calculated in our work may be of interest in the near future.