  We report the experimental realization of a single-atom heat engine. An ion
is confined in a linear Paul trap with tapered geometry and driven thermally by
coupling it alternately to hot and cold reservoirs. The output power of the
engine is used to drive a harmonic oscillation. From direct measurements of the
ion dynamics, we determine the thermodynamic cycles for various temperature
differences of the reservoirs. We use these cycles to evaluate power $P$ and
efficiency $\eta$ of the engine, obtaining up to $P=342\,$yJ and $\eta=0.28
\,\%$, consistent with analytical estimations. Our results demonstrate that
thermal machines can be reduced to the ultimate limit of single atoms.
