A new transition metal-antimony oxo-cluster based compound has been synthesized in water under room temperature. Its formula is Na₆[Cu₆MnSb₆(μ₃-OH)₂(OH)(μ₄-O)₆(tartrate)₆]·20H₂O (1), where tartrate represents rac-tartaric acid. It was characterized by elemental analysis, infrared spectrum and X-ray single-crystal diffraction. The compound crystallizes in the monoclinic system, space group P2₁/n. Structural analyses revealed that two Sb₃(μ₃-O)(tartrate)₃ scaffolds sandwich a Cu^II₆Mn^III middle layer to form the cluster. In the middle layer, all the seven metal ions lie in an almost regular hexagon, with Mn^III ion in the center and six Cu^II ions along the edges of the hexagon. As a 4-connected node, each cluster is interlinked to its nearest four {Cu₆Mn} neighbors through Na⁺, generating a 3D supramolecular framework. The temperature-dependent magnetic susceptibilities indicated dominating antiferromagnetic interactions in 1 with J_Cu–Cu = 176.34 and J_Cu–Mn = –14.44 cm⁻¹.