The dark axion portal provides a well-motivated framework for new physics beyond the Standard Model (SM), in which an axion-like particle (ALP) couples simultaneously to a photon and a dark photon, leading to distinctive collider signatures. In this work, we investigate the sensitivity to the a–γ–γ‘ interaction via the mono-photon channel arising from µ+µ− → aγ‘ with a → γγ’ at the 1.5 TeV muon collider with an integrated luminosity of L = 500 fb−1 for the polarized and unpolarized beam configurations, P(µ+,µ−) = (−100%,+100%) and P(µ+,µ−) = (0,0), using detector-level simulation. The signal is characterized by a single energetic photon accompanied by missing energy as the dark photons escape detection. The projected sensitivity reaches the O(10−5) GeV−1 level for ALP masses in the range 1 GeV ≤ ma ≤ 1200 GeV for P(µ+,µ−) = (−100%,+100%), while for P(µ+,µ−) = (0,0) the projected sensitivity reaches the O(10−4) GeV−1 level for the same ALP mass range. These results demonstrate that the mono-photon signature at a muon collider provides a sensitive probe of the a–γ–γ’ interaction over a wide mass range.