Axions and axionlike particles (ALPs) are compelling candidates for dark matter and new physics beyond the Standard Model. Photon-axion mixing in external magnetic fields not only modifies the photon energy spectrum and linear polarization state but also induces circular polarization signals. Compared to spectral and linear polarization methods, circular polarization benefits from lower astrophysical background contamination and weaker dependence on the intrinsic source spectrum, providing an independent probe for axion searches. In this work, we study the circular polarization induced by photon-axion mixing within the chiral basis framework. By analytically solving the evolution equations under the single-domain approximation, we derive an expression for the circular polarization degree $P_C$, applicable in the resonant, strong coupling, and weak coupling regimes. The opposite-phase coupling of the axion field to left- and right-handed circular polarization components generates phase differences and intensity asymmetries, thereby converting initially linearly polarized light into nonzero circular polarization signals. Within single-domain magnetic field models, we compare the energy-dependent circular polarization in four astrophysical environments (active galactic nucleus jets, the intracluster medium, the intergalactic medium, and the Galactic magnetic fields). We find that the x-ray to MeV band represents the most sensitive observational window for axion-induced circular polarization signals. In multidomain propagation models, using the blazar S4 $0954+65$ as a case study, phase accumulation in random magnetic domains causes the circular polarization degree to fluctuate with redshift and exhibit pronounced energy structures in the x-ray to MeV band. Using the optical circular polarization upper limit $P_C<0.184\%$ (measured in the z-SDSS band) from this source, we statistically constrain $g_{a\gamma \gamma }\lesssim 3\times {10}^{-11}{\mathrm{GeV}}^{-1}$ (95% confidence level) for $m_a\sim {10}^{-16}-{10}^{-10}\mathrm{eV}$, with the strongest constraint reaching $g_{a\gamma \gamma }\sim 6\times {10}^{-12}{\mathrm{GeV}}^{-1}$ near $m_a\sim {10}^{-14}\mathrm{eV}$. These results establish circular polarization as a complementary axion probe, and future high-energy circular polarization observations are expected to further strengthen constraints on the ultralight axion parameter space.

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• Particle astrophysics
• Axion-like particles
• Polarization