[en] Supernova remnants (SNRs) are commonly believed to be the primary sources of Galactic cosmic rays. Despite intensive study of the non-thermal emission of many SNRs the identification of the accelerated particle type relies heavily on assumptions of ambient-medium parameters that are only loosely constrained. Compelling evidence of hadronic acceleration can be provided by detecting a strong roll-off in the secondary γ-ray spectrum below the ${\mathrm{\pi <!--\; ??\; -->}}^0$ production threshold energy of about 135 MeV, the so called “pion bump.” Here we use five years of Fermi-Large Area Telescope data to study the spectrum above 60 MeV of the middle-aged SNR W51C. A clear break in the power-law γ-ray spectrum at $E_{\mathrm{b}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{k}}=290\pm <!--\; ??\; -->20\mathrm{M}\mathrm{e}\mathrm{V}$ is detected with $9\mathrm{\sigma <!--\; ??\; -->}$ significance and we show that this break is most likely associated with the energy production threshold of ${\mathrm{\pi <!--\; ??\; -->}}^0$mesons. A high-energy break in the γ-ray spectrum at about 2.7 GeV is found with $7.5\mathrm{\sigma <!--\; ??\; -->}$ significance. The spectral index at energies beyond this second break is ${\mathrm{\Gamma <!--\; ??\; -->}}_2={2.52}_{-<!--\; ???\; -->0.07}^{+0.06}$ and closely matches the spectral index derived by the MAGIC Collaboration above 75 GeV. Therefore our analysis provides strong evidence to explain the γ-ray spectrum of W51C by a single particle population of protons with a momentum spectrum best described by a broken power law with break momentum $p_{\mathrm{b}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{k}}\sim <!--\; ???\; -->80\mathrm{G}\mathrm{e}\mathrm{V}/c.$ W51C is the third middle-aged SNR that displays compelling evidence for cosmic-ray acceleration and thus strengthens the case of SNRs as the main source of Galactic cosmic rays.