The structural behaviour of lawsonite CaAl2Si2O7(OH)2.H2O, has been studied under quasi-hydrostatic conditions in a diamond-anvil cell to 18 GPa at room temperature, using angle-dispersive X-ray powder diffraction and Raman spectroscopy. With increasing pressure, we observe a phase transition at P = 8.6(3) GPa, characterized by (1) the splitting of diffraction lines, (2) the emergence of new Raman bands, and (3) significant changes in the frequency shifts of the hydroxyl O-H stretching modes. The transition is displacive and fully reversible, without any detectable hysteresis. The high-pressure phase, referred to here as lawsonite III can be indexed into a monoclinic unit-cell, with a = 5.6833(3), b = 8.5944(4), c = 12.8773(5) Å, Y= 91.42(4)° and V = 628.80(4) Å3 at P = 10.6 GPa. The space-group of lawsonite III is likely to be C1121/m, which is the unconventional representation of P21/m related to the low-pressure Cmcm symmetry. Assuming the change in space-group from Cmcm to C1121/m, the resulting components of the spontaneous strain tensor are analyzed in terms of the change in point group from mmm to 2/m. The pressure-dependence of the fourth power of the symmetry-breaking component e64 is linear, indicating a tricritical character for the transition. Both X-ray diffraction and Raman spectroscopy indicate that the overall aluminosilicate framework of lawsonite is retained through the transition. However, monoclinic lawsonite III is ∼ 40% less compressible than the low-pressure orthorhombic polymorph. The Raman spectroscopic results, in good agreement with recent infrared ones reveal that this decrease in compressibility is likely to be related to the increase in the hydrogen bond strength involving the hydroxyl groups in the structure, whereas the hydrogen bond system around the water molecule does not appear to be modified at the transition.