The phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) was investigated under hydrostatic pressures up to 300 MPa using wide-angle X-ray diffraction equipped with a high-pressure vessel. The reversible transitions of HHTT i.e., crystal (Cr)-helical phase (H)-hexagonal columnar phase (Col(h))-isotropic liquid (I), were observed under pressures up to about 30 MPa. The transition sequence changed significantly depending on the applied pressure. The H phase appeared monotropically in the pressure region between 50 and 180 MPa, indicating the Cr-->Col(h)-->I heating and I-->Col(h)-->H-->Cr transition sequences on cooling. Applying higher pressures altered further the phase behaviour: the enantiotropic Col(h) phase changed to monotropic one under high pressures above 180 MPa, indicating the Cr-->I on heating and I-->Col(h)-->Cr transition sequences on cooling. The X-ray diffraction pattern of the H phase exhibited a sharp low-angle and a wide-angle reflections corresponding to the intercolumnar and the core-core distances, respectively. This feature is indicative of the three-dimensional ordering present in the H phase. On the other hand, the Col(h) phase showed only a low-angle reflection. It was found that the intercolumnar distance in the Col(h) phase decreases with increasing temperature at high pressures. This indicates that the negative coefficient of thermal expansion for the column packing in the Col(h) phase is held under pressure.