  We present the analysis and the results of a 20 ks XMM-Newton observation of
the extremely X-ray loud (L_X ~ 5 x 10^{47} erg/s) flat-spectrum radio quasar
RBS 315 at a redshift of 2.69. This EPIC observation has allowed us to strongly
constrain the slope of the continuum (Gamma = 1.23+/-0.01) as well as to
discover the presence of a sharp drop below ~ 2 keV in its spectrum. Such a
flat photon index and the huge luminosity suggest that the X-ray emission is
due to the low energy tail of the Comptonized spectrum, produced from plasma in
a relativistic jet oriented close to our line of sight. Even though the
hypothesis of a break in the continuum cannot be completely discarded as an
explanation of the soft X-ray cutoff, the presence of intrinsic absorption
appears more plausible. Spectral fits with cold (Nh(z) = 1.62+/-0.09 x 10^{22}
cm^{-2}) and lukewarm (Nh(z) = 2.2^{+0.9}_{-0.3} x 10^{22} cm^{-2}; xi =
15^{+38}_{-12} erg/cm^{2}/s) absorbers are statistically indistinguishable.
Remarkably, our results are very similar to those reported so far for other
absorbed high-z Blazars observed by XMM-Newton. The existence of this
``homogeneous'' class of jet-dominated superluminous obscured QSOs at high z
therefore could be important in the context of the formation and cosmological
evolution of radio-loud objects
