  CTB 80 (G69.0+2.7) is a relatively old (50--80 kyr) supernova remnant (SNR)
with a complex radio morphology showing three extended radio arms and a radio
and X-ray nebula near the location of the pulsar PSR B1951+32. We report on a
study of the GeV emission in the region of CTB 80 with \emph{Fermi}-LAT data.
An extended source with a size of 1.3$^\circ$, matching the size of the
infrared shell associated to the SNR, was discovered. The GeV emission,
detected up to an energy of $\sim 20$ GeV, is more significant at the location
of the northern radio arm where previous observations imply that the SNR shock
is interacting with ambient material. Both hadronic and leptonic scenarios can
reproduce the multiwavelength data reasonably well. The hadronic cosmic ray
energy density required is considerably larger than the local Galactic value
and the gamma-ray leptonic emission is mainly due to bremsstrahlung
interactions. We conclude that GeV particles are still trapped or accelerated
by the SNR producing the observed high-energy emission when interacting with
ambient material.
