  We present XMM-Newton observations of the radio pulsar PSR J1119-6127, which
has an inferred age of 1,700 yr and surface dipole magnetic field strength of
4.1x10^13 G. We report the first detection of pulsed X-ray emission from PSR
J1119-6127. In the 0.5--2.0 keV range, the pulse profile shows a narrow peak
with a very high pulsed fraction of (74 +/- 14)%. In the 2.0--10.0 keV range,
the upper limit for the pulsed fraction is 28% (99% confidence). The pulsed
emission is well described by a thermal blackbody model with a temperature of
T^{\infty} = 2.4^{+0.3}_{-0.2}x10^6 K and emitting radius of 3.4^{+1.8}_{-0.3}
km (at a distance of 8.4 kpc). Atmospheric models result in problematic
estimates for the distance/emitting area. PSR J1119-6127 is now the radio
pulsar with smallest characteristic age from which thermal X-ray emission has
been detected. The combined temporal and spectral characteristics of this
emission are unlike those of other radio pulsars detected at X-ray energies and
challenge current models of thermal emission from neutron stars.
