Micro-Raman spectroscopy is applied to study structural changes in the vicinity of the domain walls in ferroelastic lead phosphate Pb3(PO4)2. The Raman spectra measured in untwinned and in heavily twinned regions of monoclinic Pb3(PO4)2 are studied and the observed differences are analysed on the basis of calculated vibrational modes of monoclinic structural units with different atomic arrangements. The wall-related Raman signals observed near 537, 65 and 51 cm−1 result from static displacements of the Pb2 atoms along the monoclinic binary axis. PO4 tetrahedra of rhombohedral geometry can also contribute to the extra Raman scattering near 537 cm−1. The deviations of the Pb2 atoms from their monoclinic positions couple preferably within the Pb2–O sheets parallel to the cleavage plane. Rearrangements of the Pb2 atoms occurring across the domain walls affect the intensity ratios between out-of-plane and in-plane components of ν4(PO4) bending, PO4 rotational and PO4 translational modes.