In the presence of a background magnetic field, axions or axionlike particles (ALPs) can be resonantly converted to photons when their mass is nearly equal to the effective photon mass. In this paper, we propose a novel method to constrain the parameter space of ALPs by investigating the resulting imprints of axion-photon conversion in the cosmic microwave background (CMB) observations. We show that a helical magnetic field existing prior to the CMB epoch can generate an excess population of photons carrying net circular polarization due to the axion-photon conversion mechanism. Consequently, current measurements of the angular power spectrum of circular polarization ($V$ mode) in the CMB can be used to constrain the parameter space of ALP mass and its coupling to photons. In the optimistic scenario of a maximally helical magnetic field with strength $\sim \mathrm{nG}$, we find that CLASS observations at 40 GHz provide the most stringent upper limits to date, on the previously unconstrained regions of axion-photon coupling corresponding to ALP masses in the range ${10}^{-10}-{10}^{-8}\mathrm{eV}$.

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• Cosmic microwave background
• Axion-like particles