NMR, muon spin rotation (μ SR), magnetization, and specific-heat measurements in Li 2 VOSiO 4 powders and single crystals are reported. Specific-heat and magnetization measurements evidence that Li 2 VOSiO 4 is a frustrated two-dimensional S= 1/2 Heisenberg antiferromagnet on a square lattice with a superexchange coupling J 1, along the sides of the square, almost equal to J 2, the one along the diagonal (J 2/J 1= 1.1±0.1 with J 2+ J 1= 8.2±1 K). At T c≃ 2.8 K a phase transition to a low-temperature collinear order is observed. T c and the sublattice magnetization, derived from NMR and μ SR, were found practically independent on the magnetic field intensity up to 9 T. The critical exponent of the sublattice magnetization was estimated β≃ 0.235, nearly coincident with the one predicted for a two-dimensional XY system on a finite size. The different magnetic properties found above and below T c are associated with the modifications in the spin Hamiltonian arising from a structural distortion occurring just above T c.