The title complex was synthesized in an aqueous solution and its crystal structure was determined by X-ray diffraction. The crystal is rhombohedral, space group R3 with a=1.947 0(3) nm, b=1.947 0(3) nm, c=1.768 3(3) nm, α=90°, β=90°, γ=120°, V=5.805(2) nm3, Z=3, the final R value is 0.051 2. The complex contains a novel tetranuclear cation with four Er3+ ions connected by four μ3-OH ions and six bridging carboxyl groups. Each erbium ion is coordinated by three μ3-OH, three carboxylic oxygen atoms from three L-proline, and two or one oxygen atoms from water molecules.