Over the last few years, air lasing has emerged as an attractive topic in standoff detection research. Until now however, existing approaches have relied on the major species of air as the gain medium. In this work, we present the first experimental demonstration of an air laser based on water vapor, a minor constituent of air. The approach relies on photodissociation of water vapor molecules followed by two-photon pumping of the constituent hydrogen atoms to produce lasing on the well-known Balmer-alpha line. We characterize this hydrogen laser in both the forward as well as return direction, and discuss the effect of some parameters, including the presence and type of buffer gas, as well as the focusing. We find that most of our results can be explained in terms of their influence on the gain of the stimulated emission process. This method can be potentially applied to other hydrogen-containing molecules and offers a promising approach for standoff detection of trace species.