A measurement is presented of a ratio observable that provides a measure of the azimuthal correlations among jets with large transverse momentum p_T. This observable is measured in multijet events over the range of p_T=360–3170GeV based on data collected by the CMS experiment in proton-proton collisions at a centre-of-mass energy of 13TeV, corresponding to an integrated luminosity of 134fb^-1. The results are compared with predictions from Monte Carlo parton-shower event generator simulations, as well as with fixed-order perturbative quantum chromodynamics (pQCD) predictions at next-to-leading-order (NLO) accuracy obtained with different parton distribution functions (PDFs) and corrected for nonperturbative and electroweak effects. Data and theory agree within uncertainties. From the comparison of the measured observable with the pQCD prediction obtained with the NNPDF3.1 NLO PDFs, the strong coupling at the Z boson mass scale is α_S(m_Z)=0.1177±0.0013(exp)_-0.0073^+0.0116(theo)=0.1177_-0.0074^+0.0117, where the total uncertainty is dominated by the scale dependence of the fixed-order predictions. A test of the running of α_S in the TeV region shows no deviation from the expected NLO pQCD behaviour.