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Observing gravitationally lensed objects in the time domain is difficult, andwell-observed time-varying sources are rare. Lensed gamma-ray bursts (GRBs)offer improved timing precision to this class of objects complementingobservations of quasars and supernovae. The rate of lensed GRBs is highlyuncertain, approximately 1 in 1000. The Gamma-ray Burst Monitor (GBM) onboardthe Fermi Gamma-ray Space Telescope has observed more than 3000 GRBs making itan ideal instrument to uncover lensed bursts. Here we present observations ofGRB 210812A showing two emission episodes, separated by 33.3 s, and with fluxratio of about 4.5. An exhaustive temporal and spectral analysis shows that thetwo emission episodes have the same pulse and spectral shape, which poseschallenges to GRB models. We report multiple lines of evidence for agravitational lens origin. In particular, modeling the lightcurve using nestedsampling we uncover strong evidence in favor of the lensing scenario. Assuminga point mass lens, the mass of the lensing object is about 1 million solarmasses. High-resolution radio imaging is needed for future lens candidates toderive tighter constraints.

Fermi-GBM Observations of GRB 210812A: Signatures of a Million Solar Mass Gravitational Lens