We report the discovery of a compact supercluster structure at z = 0.9 .
The structure comprises three optically-selected clusters , all of which are detected in X-rays and spectroscopically confirmed to lie at the same redshift .
The Chandra X-ray temperatures imply individual masses of \sim 5 \times 10 ^ { 14 } M _ { \odot } .
The X-ray masses are consistent with those inferred from optical–X-ray scaling relations established at lower redshift .
A strongly-lensed z \sim 4 Lyman break galaxy behind one of the clusters allows a strong-lensing mass to be estimated for this cluster , which is in good agreement with the X-ray measurement .
Optical spectroscopy of this cluster gives a dynamical mass in good agreement with the other independent mass estimates .
The three components of the RCS2319+00 supercluster are separated from their nearest neighbor by a mere < 3 Mpc in the plane of the sky and likely < 10 Mpc along the line-of-sight , and we interpret this structure as the high-redshift antecedent of massive ( \sim 10 ^ { 15 } M _ { \odot } ) z \sim 0.5 clusters such as MS0451.5-0305 .