A future Galactic supernova (SN) explosion can lead to a gamma-ray signal induced by ultralight axionlike particles (ALPs) thermally produced in the SN core and converted into high-energy photons in the Galactic magnetic field. The detection of such a signal is in the reach of the Large Area Telescope aboard the Fermi Gamma-Ray Space Telescope. The observation of gamma-ray emission from a future SN has a sensitivity to gaγ≳4×10−13 GeV−1 for a SN at fiducial distance of 10 kpc and would allow us to reconstruct the ALP-photon coupling within a factor of ∼2, mainly due to the uncertainties on the modeling of the Galactic magnetic field.