The ruthenium(II) complex cis‐[RuCl2(dmso)4] is known for its antiproliferative properties. This has stimulated the discovery of a wide group of new ruthenium complexes considered as potential anticancer drugs. The stability of these ruthenium complexes under physiological conditions and their interaction with protein amino acid residues are particularly important because of their intravenous administration. The studies presented here are devoted to the stability of cis‐[RuCl2(dmso)4] under physiological pH conditions and its reactivity towards L‐methionine. Immediate dissociation of one dmso ligand from the parent complex after dilution in water leads to the formation of fac‐[RuCl2(H2O)(dmso)3], which under basic conditions undergoes stepwise dissociation of chloride ions without further dmso release. The hydrolysis of fac‐[RuCl2(H2O)(dmso)3] at pH 7.4 and also base‐catalyzed hydrolysis were investigated in detail by application of kinetic and spectroscopic (UV/Vis, NMR) measurements. The combined experimental and theoretical study revealed that L‐methionine coordinates to fac‐[RuCl(OH)(H2O)(dmso)3] by substituting a water ligand with simultaneous dmso release. Detailed NMR characterization of the product indicated that methionine coordinates through the NH2 group rather than the thioether moiety. This conclusion was further supported by theoretical calculations with the application of ETS–NOCV analysis.