  We derive partial-wave unitarity constraints on gauge-invariant interactions
of an Axion-Like Particle (ALP) up to dimension-6 from all allowed $2\to2$
scattering processes in the limit of large center-of-mass energy. We find that
the strongest bounds stem from scattering amplitudes with one external ALP and
only apply to the coupling to a pair of $SU(2)_L$ gauge bosons. Couplings to
$U(1)_Y$ and $SU(3)_C$ gauge bosons and to fermions are more loosely
constrained.
