  HESS J1857+026 is an extended TeV gamma-ray source discovered by H.E.S.S.
very close to the Galactic plane. Located in the vicinity of the pulsar PSR
J1856+0245, the source represents a pulsar wind nebula (PWN) candidate. In
Fermi-LAT data, 7 photons above 100GeV were associated to it as VHE J1857+0252
by Neronov and Semikoz (2010), while in the previous MeV-GeV catalogs no
associated source was reported yet. MAGIC was upgraded to a stereoscopic
Cherenkov telescope system in 2009, which substantially improved its
performance with respect to extended objects. We observed HESS J1857+026 in
2010 and analysed 29 hours of good quality stereoscopic data, yielding a highly
significant detection. We present an energy spectrum from 100GeV to 10TeV and
skymaps for two different energy regimes. The spectrum does not show any
indication for an inverse Compton turnover, while we find an intrinsic
extension of 0.22 \pm 0.02stat \pm 0.02sys in the energy range of 200 - 1000
GeV. We discuss the possible PWN nature of the source and the performance of
MAGIC with respect to extended sources.
