Reversible switching of single-molecule magnet (SMM) behaviors was demonstrated by structural transformation of dinuclear dysprosium cores in polyoxometalates [{Dy(H₂O)₂(CH₃COCH₃)}₂(γ-SiW₁₀O₃₆)₂]¹⁰⁻ (1) and [Dy₂(μ₂-OH)₂(γ-SiW₁₀O₃₆)₂]¹²⁻ (2). Whereas 1 hardly showed a SMM behavior, 2 with bis(μ₂-OH) bridging ligands between two dysprosium cations showed a SMM behavior with a thermal energy barrier ΔE/k_B of 65.7 K. This is mainly because of large magnetic anisotropy in 2 obtained by the design of a weak interaction between ligands and the oblate-shaped electron density of dysprosium cations. The SMM behaviors could reversibly be manipulated by coordination and elimination of bis(μ₂-OH) bridging ligands and transformation of the coordination geometry around dysprosium cations between distorted monocapped trigonal prism (1) and distorted trigonal prism (2).