The title compound, [RuCl(2)(C(9)H(12))(C(2)H(6)OS)], features a planar [maximum deviation = 0.0075 (17) Å] η(6)-bound mesitylene ligand and a dimethyl sulfoxide ligand coordinated via the S atom. The overall complex geometry about the Ru(II) atom is best described as a piano-stool configuration.