Synthetic mineral libethenite Cu2PO4OH was prepared by the hydrothermal method, and its structure at 200 K was refined by single-crystal X-ray diffraction. The structure of Cu2PO4OH is built up from Cu2(2)O(6)(OH)(2) dimers of edge-sharing Cu2O(4)(OH) trigonal bipyramids and [Cu1(2)O(6)(OH)(2)](infinity) chains of edge-sharing Cu1O(4)(OH)(2) octahedra. Magnetic properties of Cu2PO4OH were investigated by magnetic susceptibility, magnetization, and specific heat measurements. Cu2PO4OH is a spin-gap system with a spin gap of about 139 K. It was shown by spin dinner analysis that, to a first approximation, the magnetic structure of Cu2PO4OH is described by an isolated square-spin cluster model defined by the Cu1-O-Cu2 superexchange J with Cu1 center dot center dot center dot Cu2 = 3.429 angstrom. The fitting analysis of the magnetic susceptibility data with a square-spin cluster model results in J/k(B) = 138 K. Specific heat data show that Cu2PO4OH does not undergo a long-range magnetic ordering down to 1.8 K. We also report vibrational properties studied with Raman spectroscopy and the thermal stability Of Cu2PO4OH.