  A measurement of the rapidity and transverse momentum dependence of dijet
azimuthal decorrelations is presented, using the quantity $R_{\Delta \phi}$.
The quantity $R_{\Delta \phi}$ specifies the fraction of the inclusive dijet
events in which the azimuthal opening angle of the two jets with the highest
transverse momenta is less than a given value of the parameter $\Delta
\phi_\mathrm{max}$. The quantity $R_{\Delta \phi}$ is measured in
proton--proton collisions at $\sqrt{s}=$8 TeV as a function of the dijet
rapidity interval, the event total scalar transverse momentum, and $\Delta
\phi_\mathrm{max}$. The measurement uses an event sample corresponding to an
integrated luminosity of 20.2 fb$^{-1}$ collected with the ATLAS detector at
the CERN Large Hadron Collider. Predictions of a perturbative QCD calculation
at next-to-leading order in the strong coupling with corrections for
non-perturbative effects are compared to the data. The theoretical predictions
describe the data in the whole kinematic region. The data are used to determine
the strong coupling $\alpha_{\mathrm{S}}$ and to study its running for momentum
transfers from 260 GeV to above 1.6 TeV. An analysis that combines data at all
momentum transfers results in $\alpha_{\mathrm{S}}(m_{Z}) =
0.1127^{+0.0063}_{-0.0027}$.
