We study the QCD axion arising from the 5th component of a bulk U(1) gauge field in a five-dimensional warped grand unified theory, and determine the viable range of the axion decay constant fa. Unlike flat extra dimensions, where gauge couplings run quickly above the Kaluza-Klein (KK) scale, the logarithmic running in warped geometries permits substantially smaller fa while preserving perturbative gauge coupling unification. However, bulk tree-level contributions to the gauge coupling — interpreted holographically as CFT renormalization — place a lower bound on fa. We find that the conventional QCD axion window 109 GeV ≲ fa ≲ 1012 GeV is readily compatible without losing perturbativity, provided the AdS curvature is near the Planck scale. Thus, the 5D warped orbifold GUT naturally accommodates a high-quality QCD axion in a grand unified theory that provides an effective description of string-theoretic warped flux compactifications, admitting complementary geometric and holographic descriptions of the axion.
