  The gradient of the Casimir force between a Si-SiO${}_2$-graphene substrate
and an Au-coated sphere is measured by means of a dynamic atomic force
microscope operated in the frequency shift technique. It is shown that the
presence of graphene leads to up to 9% increase in the force gradient at the
shortest separation considered. This is in qualitative agreement with the
predictions of an additive theory using the Dirac model of graphene.
