  We report the discovery of a UHE gamma-ray source, LHAASO J2108+5157, by
analyzing the LHAASO-KM2A data of 308.33 live days. Significant excess of
gamma-ray induced showers is observed in both energy bands of 25-100 TeV and
$\gt$100 TeV with 9.5 sigma and 8.5 sigma, respectively. This source is not
significantly favored as an extensive source with the angular extension smaller
than the point-spread function of KM2A. The measured energy spectrum from 20 to
200 TeV can be approximately described by a power-law function with an index of
-2.83$\pm$ 0.18stat. A harder spectrum is demanded at lower energies
considering the flux upper limit set by Fermi-LAT observations. The position of
the gamma-ray emission is correlated with a giant molecular cloud, which favors
a hadronic origin. No obvious counterparts have been found, deeper
multiwavelength observations will help to shed new light on this intriguing UHE
source.
