  The very high energy (VHE) \gamma-ray source HESS J0632+057 has recently been
confirmed to be a \gamma-ray binary. The optical counterpart is the Be star MWC
148, and a compact object of unknown nature orbits it every ~321 d with a high
eccentricity of ~0.8. We monitored HESS J0632+057 with the stereoscopic MAGIC
telescopes from 2010 October to 2011 March and detected significant VHE
\gamma-ray emission during 2011 February, when the system exhibited an X-ray
outburst. We find no \gamma-ray signal in the other observation periods when
the system did not show increased X-ray flux. Thus HESS J0632+057 exhibits
\gamma-ray variability on timescales of the order of one to two months possibly
linked to the X-ray outburst that takes place about 100 days after the
periastron passage. Furthermore our measurements provide for the first time the
\gamma-ray spectrum down to about 140 GeV and indicate no turnover of the
spectrum at low energies. We compare the properties of HESS J0632+057 with the
similar \gamma-ray binary LS I +61 303, and discuss on the possible origin of
the multi-wavelength emission of the source
