Context :

Aims : Probe the high-energy ( > 60 MeV ) emission from the black hole X-ray binary system , Cygnus X-1 , and investigate its origin .

Methods : We analysed 7.5 yr of data by Fermi -LAT with the latest Pass 8 software version .

Results : We report the detection of a signal at \sim 8 \sigma statistical significance spatially coincident with Cygnus X-1 and a luminosity above 60 MeV of 5.5 \times 10 ^ { 33 } erg s ^ { -1 } .
The signal is correlated with the hard X-ray flux : the source is observed at high energies only during the hard X-ray spectral state , when the source is known to display persistent , relativistic radio emitting jets .
The energy spectrum , extending up to \sim 20 GeV without any sign of spectral break , is well fitted by a power-law function with a photon index of 2.3 \pm 0.2 .
There is a hint of orbital flux variability , with high-energy emission mostly coming around the superior conjunction .

Conclusions : We detected GeV emission from Cygnus X-1 and probed that the emission is most likely associated with the relativistic jets .
The evidence of flux orbital variability points to the anisotropic inverse Compton on stellar photons as the mechanism at work , thus constraining the emission region to a distance 10 ^ { 11 } -10 ^ { 13 } cm from the black hole .