  The existence of a "knee" at energy ~1 PeV in the cosmic-ray spectrum
suggests the presence of Galactic PeV proton accelerators called "PeVatrons".
Supernova Remnant (SNR) G106.3+2.7 is a prime candidate for one of these. The
recent detection of very high energy (0.1-100 TeV) gamma rays from G106.3+2.7
may be explained either by the decay of neutral pions or inverse Compton
scattering by relativistic electrons. We report an analysis of 12 years of
Fermi-LAT gamma-ray data which shows that the GeV-TeV gamma-ray spectrum is
much harder and requires a different total electron energy than the radio and
X-ray spectra, suggesting it has a distinct, hadronic origin. The non-detection
of gamma rays below 10 GeV implies additional constraints on the relativistic
electron spectrum. A hadronic interpretation of the observed gamma rays is
strongly supported. This observation confirms the long-sought connection
between Galactic PeVatrons and SNRs. Moreover, it suggests that G106.3+2.7
could be the brightest member of a new population of SNRs whose gamma-ray
energy flux peaks at TeV energies. Such a population may contribute to the
cosmic-ray knee and be revealed by future very high energy gamma-ray detectors.
