Molecular machines, and in particular molecular motors with synthetic molecular structures and fuelled by external light, voltage or chemical conversions, have recently been reported. 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). 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 or indirect signatures 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.