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Since its launch, the Fermi satellite has firmly identified 5 pulsar windnebulae plus a large number of candidates, all powered by young and energeticpulsars. HESS J1857+026 is a spatially extended gamma-ray source detected byH.E.S.S. and classified as a possible pulsar wind nebula candidate powered byPSR J1856+0245. We search for gamma-ray pulsations from PSR J1856+0245 andexplore the characteristics of its associated pulsar wind nebula. Using arotational ephemeris obtained from the Lovell telescope at Jodrell BankObservatory at 1.5 GHz, we phase-fold 36 months of gamma-ray data acquired bythe Large Area Telescope (LAT) aboard Fermi. We also perform a completegamma-ray spectral and morphological analysis. No gamma-ray pulsations weredetected from PSR J1856+0245. However, significant emission is detected at aposition coincident with the TeV source HESS J1857+026. The gamma-ray spectrumis well described by a simple power-law with a spectral index of 1.53 \pm0.11_{\rm stat} \pm 0.55_{\rm syst}$ and an energy flux of $G(0.1$--100GeV$)=(2.71 \pm 0.52_{\rm stat} \pm 1.51_{\rm syst}) \times 10^{-11}$ ergscm$^{-2}$ s$^{-1}$. The $\gamma$-ray luminosity is $L_{PWN}^{\gamma} (0.1$--100GeV$)=(2.5 \pm 0.5_{stat} \pm 1.5_{syst}) \times 10^{35} (\frac{d}{9 kpc})^2$ergs s$^{-1}$, assuming a distance of 9 kpc. This implies a $\gamma-$rayefficiency of $\sim$ 5% for $\dot{E}=4.6 \times 10^{36}$ erg $s^{-1}$, in therange expected for pulsar wind nebulae. Detailed multi-wavelength modelingprovides new constraints on its pulsar wind nebula nature.

Fermi-LAT constraints on the Pulsar Wind Nebula nature of HESS J1857+026