We report on the effects of a high hydrostatic pressure on the intra-molecular modes of a sexithiophene (T6) single crystal. Raman experiments were performed at low temperature up to a pressure of 80 kbar. We find that intermolecular interaction gives rise to a sizeable redistribution of the intensities of the strongest intra-molecular Raman lines. These changes are due to a slight deformation occurring in the terminal rings of the sexithiophene molecule. The frequencies of the bending modes involving sulfur atoms are the most affected by pressure. A Davydov doublet and a mixed inter/intra-molecular mode are observed. These results confirm the role played by the sulfur atoms in modulating the electronic overlap between chains.