  We present a Hubble Space Telescope image of the FRII radio galaxy 3C 401,
obtained at 1.6 microns with the NICMOS camera in which we identify the
infrared counterpart of the brightest region of the radio jet. The jet has a
complex radio structure and brightens where bending occurs, most likely as a
result of relativistic beaming. We analyze archival data in the radio, optical
and X-ray bands and we derive its spectral energy distribution. Differently
from all of the previously known optical extragalactic jets, the jet in 3C401
is not detected in the X-rays even in a long 48ksec X-ray Chandra exposure and
the infrared emission dominates the overall SED. We propose that the dominant
radiation mechanism of this jet is synchrotron. The low X-ray emission is then
caused by two different effects: i) the lack of any strong external photon
field and ii) the shape of the electron distribution. This affects the location
of the synchrotron peak in the SED, resulting in a sharp cut-off at energies
lower than the X-rays. Thus 3C401 shows a new type of jet which has
intermediate spectral properties between those of FRI, which are dominated by
synchrotron emission up to X-ray energies, and FRII/QSO, which show a strong
high-energy emission due to inverse-Compton scattering of external photons.
This might be a clue for the presence of a continuous ``sequence'' in the
properties of large scale jets, in analogy with the ``blazar sequence'' already
proposed for sub-pc scale jets.
