Gravity is the weakest of all known fundamental forces and poses some of the most important open questions to modern physics: it remains resistant to unification within the standard model of physics and its underlying concepts appear to be fundamentally disconnected from quantum theory.sup.1-4. Testing gravity at all scales is therefore an important experimental endeavour.sup.5-7. So far, these tests have mainly involved macroscopic masses at the kilogram scale and beyond.sup.8. Here we show gravitational coupling between two gold spheres of 1 millimetre radius, thereby entering the regime of sub-100-milligram sources of gravity. Periodic modulation of the position of the source mass allows us to perform a spatial mapping of the gravitational force. Both linear and quadratic coupling are observed as a consequence of the nonlinearity of the gravitational potential. Our results extend the parameter space of gravity measurements to small, single source masses and low gravitational field strengths. Further improvements to our methodology will enable the isolation of gravity as a coupling force for objects below the Planck mass. This work opens the way to the unexplored frontier of microscopic source masses, which will enable studies of fundamental interactions.sup.9-11 and provide a path towards exploring the quantum nature of gravity.sup.12-15. A direct measurement of the gravitational coupling between two 1-mm-radius gold spheres with masses below 100 mg using a torsional pendulum is reported.