The X-ray crystal structure of the title phthalazin-1-one derivative, C17H16N2O3S {systematic name: 2-[(2,4,6-tri-methyl-benzene)-sulfon-yl]-1,2-di-hydro-phthalazin-1-one}, features a tetra-hedral sulfoxide-S atom, connected to phthalazin-1-one and mesityl residues. The dihedral angle [83.26 (4)°] between the organic substituents is consistent with the mol-ecule having the shape of the letter V. In the crystal, phthalazinone-C6-C-H⋯O(sulfoxide) and π(phthalazinone-N2C4)-π(phthalazinone-C6) stacking [inter-centroid distance = 3.5474 (9) Å] contacts lead to a linear supra-molecular tape along the a-axis direction; tapes assemble without directional inter-actions between them. The analysis of the calculated Hirshfeld surfaces confirm the importance of the C-H⋯O and π-stacking inter-actions but, also H⋯H and C-H⋯C contacts. The calculation of the inter-action energies indicate the importance of dispersion terms with the greatest energies calculated for the C-H⋯O and π-stacking inter-actions.