  We report on an XMM-Newton observation of the most distant known quasar,
SDSSp J104433.04-012502.2, at z=5.80. We have detected this quasar with high
significance in the rest-frame 3.4-13.6 keV band, making it the most distant
cosmic object detected in X-rays; 32 +/- 9 counts were collected. SDSSp
J104433.04-012502.2 is notably X-ray weak relative to other luminous, optically
selected quasars, with alpha_ox=-1.91 +/- 0.05 and a 3.4-13.6 keV luminosity of
about 1.8 x 10^{44} erg/s. The most likely reason for its X-ray weakness is
heavy absorption with N_H greater than about 10^{24} 1/cm^2, as is seen in some
Broad Absorption Line quasars and related objects; we discuss this and other
possibilities. High-quality spectroscopy from 0.95-1.10 microns to search for
blueshifted C IV absorption may elucidate the origin of the X-ray weakness.
