  We present the first detection of X-ray coherent pulsations from the
transitional millisecond pulsar XSS J12270-4859, while it was in a sub-luminous
accretion disk state characterized by a 0.5-10 keV luminosity of 5E33 erg/s
(assuming a distance of 1.4 kpc). Pulsations were observed by XMM-Newton at an
rms amplitude of (7.7 +/- 0.5)% with a second harmonic stronger than the the
fundamental frequency, and were detected when the source is neither flaring nor
dipping. The most likely interpretation of this detection is that matter from
the accretion disk was channelled by the neutron star magnetosphere and
accreted onto its polar caps. According to standard disk accretion theory, for
pulsations to be observed the mass in-flow rate in the disk was likely larger
than the amount of plasma actually reaching the neutron star surface; an
outflow launched by the fast rotating magnetosphere then probably took place,
in agreement with the observed broad-band spectral energy distribution. We also
report about the non-detection of X-ray pulsations during a recent observation
performed while the source behaved as a rotationally-powered radio pulsar.
