Single atoms or ions on surfaces affect processes from nucleation1 to electrochemical reactions2 and heterogeneous catalysis3. Transmission electron microscopy (TEM) is a leading approach for visualizing single atoms on a variety of substrates4,5. It conventionally requires high vacuum conditions, but has been developed for in situ imaging in liquid and gaseous environments6,7 with a combined spatial and temporal resolution that is unmatched by any other method —notwithstanding concerns about electron beam effects on samples. When imaging in liquid using commercial technologies, electron scattering in the windows enclosing the sample and in the liquid generally limits the achievable resolution to a few nanometres6,8,9. Graphene liquid cells, on the other hand, have enabled atomic resolution imaging of metal nanoparticles in liquids10. Here we show that a double graphene liquid cell, comprised of a central molybdenum disulphide monolayer separated by hexagonal boron nitride spacers from the two enclosing graphene windows, makes it possible to monitor with atomic resolution the dynamics of platinum adatoms on the monolayer in an aqueous salt solution. By imaging over 70,000 single adatom adsorption sites, we compare the site preference and dynamic motion of the adatoms in both a fully hydrated and vacuum state. We find a modified adsorption site distribution and higher diffusivities for the adatoms in liquid phase compared to those in vacuum. This approach paves the way for in situ liquid phase imaging of chemical processes with single atom precision.