The X-ray crystal structure of the title phthalazin-1-one derivative, C17 H16N2O3S {systematic name: 2-[(2,4,6-trimethylbenzene)sulfonyl]-1,2-dihydrophthalazin- 1-one}, features a tetrahedral 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 molecule having the shape of the letter V. In the crystal, phthalazinone-C6-C--H…O(sulfoxide) and τ(phthala- (phthalazinone-N2 C4 )-τ(phthalazinone-C6) stacking [inter-centroid distance = 3.5474 (9) A ° ] contacts lead to a linear supramolecular tape along the a-axis direction; tapes assemble without directional interactions between them. The analysis of the calculated Hirshfeld surfaces confirm the importance of the C-- H…O and τ-stacking interactions but, also H…H and C--H…C contacts. The calculation of the interaction energies indicate the importance of dispersion terms with the greatest energies calculated for the C--H…O and τ-stacking interactions. [ABSTRACT FROM AUTHOR]