Gravity is the weakest fundamental interaction and the only one that has not been measured at the particle level. Traditional experimental methods, from astronomical observations to torsion balances, use macroscopic masses to both source and probe gravitational fields 1 . Matter wave interferometers 2 have used neutrons 3 , atoms 4 and molecular clusters 5 as microscopic test particles, but initially probed the field sourced by the entire earth. Later, the gravitational field arising from hundreds of kilograms of artificial source masses was measured with atom interferometry 6,7 . Miniaturizing the source mass and moving it into the vacuum chamber could improve positioning accuracy, allow the use of monocrystalline source masses for improved gravitational measurements, and test new physics, such as beyond-standard-model ("fifth") forces of nature and non-classical effects of gravity. In this work, we detect the gravitational force between freely falling cesium atoms and an in-vacuum, centimeter-sized source mass using atom interferometry with state-of-the-art sensitivity. The ability to sense gravitationalstrength coupling is conjectured to access a natural lower bound for fundamental forces 8 , thereby representing an important milestone in searches for physics beyond the standard model. A local, in-vacuum source mass is particularly sensitive to a wide class of interactions whose effects would otherwise be suppressed beyond detectability 9,10 in regions of high matter density. For example, our measurement strengthens limits on a number of cosmologically-motivated scalar field models, such as chameleon 10 and symmetron fields 11,12 , by over two orders of magnitude and paves the way toward novel measurements of Newton's gravitational constant G and the gravitational Aharonov-Bohm effect 13 .We measure the acceleration experienced by atoms near a miniature, in-vacuum source mass using light-pulse atom interferometry 4 . This technique is based on the wave-particle duality of quantum mechanics and transduces the acceleration experienced by atoms into a phase difference between interfering atomic matter-waves. In our setup, cesium atoms are laser-cooled and launched 14 upwards into free fall. Pulses from counter-propagating laser beams transfer them from their initial quantum state |a〉 to another state |b〉. Each atom absorbs one photon, having a momentum ℏk1, from the first beam while simultaneously being stimulated to emit another photon into the second beam, gaining additional momentum ℏk2. This results in a total momentum change of ℏkeff, where keff=k1+k2. The first interferometer pulse has a duration such that the transfer takes place with 50% probability. It acts as a coherent beam splitter for matter waves, placing the atom into a superposition of the initial state |a, p0〉 with momentum p0 and the state |b, p0+ℏkeff〉. The two states separate spatially. After a pulse separation time T, a second laser pulse transfers the states |a, p0〉 → |b, p0+ℏkeff〉 and |b, p0+ℏkeff〉 → |a, p0〉, thus inverting the relative mot...