The AtO+ molecular ion, a potential precursor for the synthesis of radiotherapeutic agents in nuclear medicine, readily reacts in aqueous solution with organic and inorganic compounds, but at first glance, these reactions must be hindered by spin restriction quantum rules. Using relativistic quantum calculations, coupled to implicit solvation models, on the most stable AtO+(H2O)6 clusters, we demonstrate that specific interactions with water molecules of the first solvation shell induce a spin change for the AtO+ ground state, from a spin state of triplet character in the gas phase to a Kramers-restricted closed-shell configuration in solution. This peculiarity allows rationalization of the AtO+ reactivity with closed-shell species in aqueous solution and may explain the differences in astatine reactivity observed in 211At production protocols based on "wet" and "dry" processes.