The title pyridazinone derivative, C21H19ClN2O3, is not planar. The unsubstituted phenyl ring and the pyridazine ring are inclined to each other, making a dihedral angle of 17.41 (13)° whereas the Cl-substituted phenyl ring is nearly orthogonal to the pyridazine ring [88.19 (13)°]. In the crystal, C—H· · ·O hydrogen bonds generate dimers with R2²(10) and R2²(24) ring motifs which are linked by C—H···O interactions, forming chains extending parallel to the c-axis direction. The intermolecular interactions were investigated using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing that the most significant contributions to the crystal packing are from H· · ·H (44.5%), C· · ·H/H· · ·C (18.5%), H· · ·O/H· · ·O (15.6%), Cl· · ·H/H· · ·Cl (10.6%) and C· · ·C (2.8%) contacts. [ABSTRACT FROM AUTHOR]