We report the experimental realization of a single-atom heat engine. An ion is con ned 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 dierences of the reservoirs. We use these cycles to evaluate power P and eciency of the engine, obtaining up to P = 342 yJ and = 2:8h, consistent with analytical estimations. Our results demonstrate that thermal machines can be reduced to the ultimate limit of single atoms.