We observe matterwave interference of a single cesium atom in free fall.The interferometer is an absolute sensor of acceleration and we show that this technique is sensitive to forces at the level of 3.2 × 10 -27 Newtons with a spatial resolution at the micron scale.We observe the build up of the interference pattern one atom at a time in an interferometer where the mean path separation extends far beyond the coherence length of the atom.Using the coherence length of the atom wavepacket as a metric, we directly probe the velocity distribution and measure the temperature in 1-D of a single atom in free fall.