Chlorine adsorption on Ru(0001) surface has been studied by a combined density functional theory (DFT) and quantitative low energy electron diffraction (LEED) approach. The (v3 × v3)R30°-Cl phase with TCl = 1/3 ML and chlorine sitting in fcc sites has been identified by DFT calculations as the most stable chlorine adsorbate structure on Ru(0001) with an adsorption energy of - 220 kJ/mol. The atomic geometry of (v3 × v3)R30°-Cl was determined by quantitative LEED. The achieved agreement between experimental and simulated LEED data is quantified by a Pendry factor of rP = 0.19 for a fcc adsorption site with a Cl-Ru bond length of 2.52 Å. At chlorine coverages beyond 1/3 ML LEED reveals diffuse diffraction rings, indicating a continuous compression of the hexagonal Cl overlayer with a preferred average Cl–Cl distance of 4.7 Å in the (v3 × v3)R30°-Cl, TCl = 1/3 ML phase towards 3.9 Å at saturation coverage of 0.48 ML.