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We apply the inverse Gertsenshtein effect, i.e., the graviton-photonconversion in the presence of a magnetic field, to constrain high-frequencygravitational waves (HFGWs). Using existing astrophysical measurements, wecompute upper limits on the GW energy densities $\Omega_{\rm GW}$ at 16different frequency bands. Given the observed magnetisation of galaxy clusterswith field strength $B\sim\mu{\rm G}$ correlated on $\mathcal{O}(10)\,{\rmkpc}$ scales, we estimate HFGW constraints in the $\mathcal{O}(10^2)\,{\rmGHz}$ regime to be $\Omega_{\rm GW}\lesssim10^{16}$ with the temperaturemeasurements of the Atacama Cosmology Telescope (ACT). Similarly, weconservatively obtain $\Omega_{\rm GW}\lesssim10^{13} (10^{11})$ in the$\mathcal{O}(10^2)\,{\rm MHz}$ ($\mathcal{O}(10)\,{\rm GHz}$) regime byassuming uniform magnetic field with strength $B\sim0.1\,{\rm nG}$ andsaturating the excess signal over the Cosmic Microwave Background (CMB)reported by radio telescopes such as the Experiment to Detect the Global EoRSignature (EDGES), LOw Frequency ARray (LOFAR), and Murchison Widefield Array(MWA), and the balloon-borne second generation Absolute Radiometer forCosmology, Astrophysics, and Diffuse Emission (ARCADE2) with graviton-inducedphotons. Although none of these existing constraints fall below the criticalvalue of $\Omega_{\rm GW} = 1$ or reaches the Big Bang Nucleosynthesis (BBN)bound of $\Omega_{\rm GW}\simeq1.2\times10^{-6}$, the upcoming Square KilometerArray (SKA) can improve the sensitivities by roughly 10 orders of magnitude andpotentially become realistic probes of HFGWs. We also explore severalnext-generation CMB surveys, including Primordial Inflation Explorer (PIXIE),Polarized Radiation Interferometer for Spectral disTortions and INflationExploration (PRISTINE) and Voyage 2050, that could potentially provideconstraints competitive to the current BBN bound.

Inverse Gertsenshtein effect as a probe of high-frequency gravitational waves