  The electronic properties of graphene have been intensively investigated over
the last decade, and signatures of the remarkable features of its linear Dirac
spectrum have been displayed using transport and spectroscopy experiments. In
contrast, the orbital magnetism of graphene, which is one of the most
fundamental signature of the characteristic Berry phase of graphene's
electronic wave functions, has not yet been measured in a single flake. In
particular, the striking prediction of a divergent diamagnetic response at zero
doping calls for an experimental test. Using a highly sensitive Giant
Magnetoresistance sensor (GMR) we have measured the gate voltage-dependent
magnetization of a single graphene monolayer encapsulated between boron nitride
crystals. The signal exhibits a diamagnetic peak at the Dirac point whose
magnetic field and temperature dependences agree with theoretical predictions
starting from the work of Mc Clure \cite{McClure1956}. Our measurements open a
new field of investigation of orbital currents in graphene and 2D topological
materials, offering a new means to monitor Berry phase singularities and
explore correlated states generated by combined effects of Coulomb
interactions, strain or moir\'e potentials.
