GaPO4 single crystals have been hydrothermally grown by the horizontal temperature gradient method. The physical properties of these single crystals were measured. The following results were obtained: lattice parametersa = 0.490 nm andc = 1.105 nm, density ϱ = 3.56 g cm−3, Vickers hardnessHv = 7.06 × 109 Nm−2, refractive indicesne = 1.611±0.006 andn0 = 1.599±0.006, and birefringence Δn = +0.012. GaPO4 single crystals exhibited some similar properties to α-quartz and AlPO4 single crystals because of the similarity of their crystal structures.