Significance Conversion of undirected energy input into directed motion on molecular scales is the basis for controlled movements in living organisms. In this context, fundamental insights can be obtained by investigating artificial molecular machines under well-defined conditions. We devised the currently smallest, atomically precise molecular machine, whose rotor (C ₂ H ₂ ) consists of just four atoms and whose functioning we have tracked employing scanning tunneling microscopy (STM). Unlike all other reported surface-anchored rotors, ours is characterized by an extremely high degree of directionality which is independent of STM-tip condition or position, therefore solely defined by the chiral support. Owing to its ultrasmall size, our rotor’s operation crosses the well-established classical to an unanticipated quantum tunneling kinetic regime without loss in directionality.