  We report the discovery of an X-ray luminous galaxy cluster at z=1.26.
RXJ0848.9+4452 was selected as an X-ray cluster candidate in the ROSAT Deep
Cluster Survey, on the basis of its spatial extent. Deep optical and near-IR
imaging have revealed a galaxy overdensity around the peak of the X-ray
emission, with a significant excess of red objects with J-K colors typical of
elliptical galaxies at z>1. Spectroscopic observations at the Keck II telescope
have secured 6 galaxy redshifts in the range 1.257<z<1.267 (<z>=1.261), within
a 35 arcsec radius around the peak X-ray emission. This system lies only 4.2
arcmin away (5.0 h^{-1}_{50} comoving Mpc, q_0=0.5) from the galaxy cluster ClG
J0848+4453, which was identified by Stanford et al. (1997) at z=1.273 in a
near-IR field galaxy survey, and is also known to be X-ray luminous. Assuming
that the X-ray emission is entirely due to hot intra-cluster gas, both these
systems have similar rest frame luminosities L_x ~=1x10^{44} ergs/s (0.5-2.0
keV band). In combination with our spectrophotometric data for the entire 30
arcmin^2 field, this suggests the presence of a superstructure, consisting of
two collapsed, possibly virialized clusters, the first detected to date at z>1.
