The title compound, 4,5-Dichloropyridazin-3-(2H)-one, has been characterized using ¹H and ¹³C NMR spectroscopic and crystallographic techniques. The compound crystal in the monoclinic space group P21/c with a= 5.22 (1) Å, b= 9.08 (2) Å, c= 12.97 (3) Å, α=γ= 90.0 (0)º, β= 100.47 (2)º and Dx= 1.808 g/cm3, respectively. The structure of the compound has also been examined by using quantum chemical methods. The molecular geometry and vibrational frequencies of monomeric and dimeric forms of the title compound in the ground state have been calculated using the Density Functional Theory (DFT) and B3LYP/6-31G(d,p) as basis set. The calculated results show that the optimized geometry and the theoretical vibration frequencies of the dimeric form are good agreement with experimental data. In addition, MEP map, HOMO, LUMO, energy gap (ΔE= EHOMO-ELUMO), electronegativity (x), hardness (η), softness (ζ), and electronegativity index (ψ) of the 4,5-dichloropyridazin-3-(2H)-one were performed at B3LYP/6-31G(d,p) level of theory.