  Recent high-redshift Type Ia supernovae results can be used to set new bounds
on a possible variation of the gravitational constant $G$. If the local value
of $G$ at the space-time location of distant supernovae is different, it would
change both the kinetic energy release and the amount of $^{56}$Ni synthesized
in the supernova outburst. Both effects are related to a change in the
Chandrasekhar mass $M_{Ch} \propto G^{-3/2}$. In addition, the integrated
variation of $G$ with time would also affect the cosmic evolution and therefore
the luminosity distance relation. We show that the later effect in the
magnitudes of Type Ia supernovae is typically several times smaller than the
change produced by the corresponding variation of the Chandrasekhar mass. We
investigate in a consistent way how a varying $G$ could modify the Hubble
diagram of Type Ia supernovae and how these results can be used to set upper
bounds to a hypothetical variation of $G$. We find $G/G_0 \la 1.1$ and $G'/G
\la 10^{-11} yr^{-1}$ at redshifts $z\simeq 0.5$. These new bounds extend the
currently available constrains on the evolution of $G$ all the way from solar
and stellar distances to typical scales of Gpc/Gyr, i.e. by more than 15 orders
of magnitudes in time and distance.
