Traditional gravity measurements use bulk masses to both source and probe gravitational fields. Matter-wave interferometers enable the use of probe masses as small as neutrons, atoms and molecular clusters, but still require fields generated by masses ranging from hundreds of kilograms^, to the entire Earth. Shrinking the sources would enable versatile configurations, improve positioning accuracy, enable tests for beyond-standard-model (‘fifth’) forces, and allow observation of non-classical effects of gravity. Here we detect the gravitational force between freely falling caesium atoms and an in-vacuum, miniature (centimetre-sized, 0.19 kg) source mass using atom interferometry. Sensitivity down to gravitational strength forces accesses the natural scale for a wide class of cosmologically motivated scalar field models^, of modified gravity and dark energy. We improve the limits on two such models, chameleons and symmetrons^,, by over two orders of magnitude. We expect further tests of dark energy theories, and measurements of Newton’s gravitational constant and the gravitational Aharonov–Bohm effect.