The highest energy photons from gamma-ray bursts (GRBs) have important implications for the physics of particle acceleration and constraints on the extragalactic background light (EBL). Here we report for the first time the detection of photons beyond 10 TeV from the unprecedentedly brightest GRB 221009A by the Large High Altitude Air-shower Observatory (LHAASO). The LHAASO-KM2A detector registered more than 140 photons with energies above 3 TeV from GRB 221009A during 230$-$900s after the trigger. The intrinsic energy spectrum of gamma-rays can be described by a power-law after correcting for EBL absorption, with the highest energy photon of 13 TeV. Such a hard spectrum challenges the synchrotron self-Compton (SSC) scenario of relativistic electrons for the afterglow emission above several TeV, because the Klein-Nishina effect and the internal $\gamma\gamma$ absorption within the GRB generally lead to a spectral softening towards high energies. Observations of photons up to 13 TeV from a source with a measured redshift of z=0.151 requires more transparency in intergalactic space than previously expected, in order to avoid an unusual pile-up at the end of the spectrum. Alternatively, one may invoke new physics such as Lorentz Invariance Violation (LIV) or assume an axion origin of very high energy (VHE) signals.
