  We report the detection of GeV $\gamma$-ray emission from supernova remnant
HESS J1731-347 using 9 years of {\it Fermi} Large Area Telescope data. We find
a slightly extended GeV source in the direction of HESS J1731-347. The spectrum
above 1 GeV can be fitted by a power-law with an index of $\Gamma =
1.77\pm0.14$, and the GeV spectrum connects smoothly with the TeV spectrum of
HESS J1731-347. Either a hadronic-leptonic or a pure leptonic model can fit the
multi-wavelength spectral energy distribution of the source. However, the hard
GeV $\gamma$-ray spectrum is more naturally produced in a leptonic (inverse
Compton scattering) scenario, under the framework of diffusive shock
acceleration. We also searched for the GeV $\gamma$-ray emission from the
nearby TeV source HESS J1729-345. No significant GeV $\gamma$-ray emission is
found, and upper limits are derived.
