  The dark photon and the axion (or axion-like particle) are popular light
particles of the hidden sector. Each of them has been actively searched for
through the couplings called the vector portal and the axion portal. We
introduce a new portal connecting the dark photon and the axion
(axion--photon--dark photon, axion--dark photon--dark photon), which emerges in
the presence of the two particles. This dark axion portal is genuinely new
couplings, not just from a product of the vector portal and the axion portal,
because of the internal structure of these couplings. We present a simple model
that realizes the dark axion portal and discuss why it warrants a rich
phenomenology.
