  The singlet pseudoscalar sector of light mesons is investigated in the
soft-wall model, a bottom-up approach to the AdS/QCD correspondence. The
$\eta^\prime$ mass results from the mixing among the fields dual to the axial
current, pseudoscalar current and the $G\tilde G$ operator. The topological
susceptibility is computed for any quark mass, and the Witten-Veneziano
relation is obtained in the large $N_c$ limit.
