  A holographic model of QCD axion is presented. It describes a composite axion
in the KSVZ class. Having a gravity dual, based on the Witten-Sakai-Sugimoto
model, it is calculable in the strongly coupled regime. Its basic properties
are derived, including the low energy Lagrangian, from which the axion
couplings to nucleons can be derived. Basic features in the deconfined phase
are studied as well. In particular, the temperature dependence of the axion
mass is extracted from the topological susceptibility. As an aside, the
topological susceptibility of strongly coupled ${\cal N}=4$ SYM at finite
temperature is derived for the first time.
