Here we report the direct observation of single electron charging of a single atomic dangling bond (DB) on the H-Si(100)-2×1 surface. The tip of a scanning tunneling microscope is placed adjacent to the DB to serve as a single-electron sensitive charge detector. Three distinct charge states of the dangling bond--positive, neutral, and negative--are discerned. Charge state probabilities are extracted from the data, and analysis of current traces reveals the characteristic single-electron charging dynamics. Filling rates are found to decay exponentially with increasing tip-DB separation, but are not a function of sample bias, while emptying rates show a very weak dependence on tip position, but a strong dependence on sample bias, consistent with the notion of an atomic quantum dot tunnel coupled to the tip on one side and the bulk silicon on the other.