Received; accepted Accepted for publication in The Astronomical Journal

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 αox = −1.91 ± 0.05 and a 3.4–13.6 keV luminosity of ≈ 1.8× 10 erg s. The most likely reason for its X-ray weakness is heavy absorption with NH > ∼ 10 24 cm, 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 μm to search for blueshifted C iv absorption may elucidate the origin of the X-ray weakness. Subject headings: galaxies: active — galaxies: nuclei — galaxies: quasars: general — galaxies: quasars: individual (SDSSp J104433.04–012502.2) — X-rays: galaxies