Using N,N'?Dimethylpiperazine as structure directing agent, a 3D open?framework neodymium oxalate [Nd(H₂O)(C₂O₄)₂](C₆N₂H₁₆)_0.5?2H₂O (compound 1) was synthesized under hydrothermal condition. The single crystal X?ray diffraction reveals that the structure is composed of alternating single?capped tetragonal antiprism NdO₉ and oxalic acid units connected to each other. The structure contains three?dimensional 12?membered ring channels, in which water molecules and protonated Me₂ppz cations are located in the channels along the b and a axes, respectively. The presence of guest water molecules provides the structural basis for proton conductivity. The results of hydrothermal and acid?base stability tests show that the structure of compound 1 had excellent stability. AC impedance tests show that compound 1 has a high proton conductivity, which is 2.81×10^-4 S/cm at 348 K and 98% RH.