Molecular machines, and in particular molecular motors with synthetic molecular structures and fuelled by external light, voltage or chemical conversions, have recently been reported(1-6). Most of these experiments are carried out in solution with a large ensemble of molecules and without access to one molecule at a time, a key point for future use of single molecular machines with an atomic scale precision. Therefore, to experiment on a single molecule-machine, this molecule has to be adsorbed on a surface, imaged and manipulated with the tip of a scanning tunnelling microscope ( STM)(7-10). A few experiments of this type have described molecular mechanisms in which a rotational movement of a single molecule is involved. However, until now, only uncontrolled rotations(11-13) or indirect signatures(14,15) of a rotation have been reported. In this work, we present a molecular rack-and-pinion device for which an STM tip drives a single pinion molecule at low temperature. The pinion is a 1.8-nm-diameter molecule functioning as a six-toothed wheel interlocked at the edge of a self-assembled molecular island acting as a rack. We monitor the rotation of the pinion molecule tooth by tooth along the rack by a chemical tag attached to one of its cogs.