Although few-layer bismuth oxyhalides (BiOX, X = Cl, Br, and I) have been shown to be appropriate for photocatalytic hydrogen production, the hydrogen evolution reaction (HER) activity of BiOX is unrevealed. Herein, the origins of catalytic activity on single-layer BiOX are investigated by using the density functional theory. The grand potential calculations show that the Bi-and BiO-terminations of single-layer BiOX are stable in O-poor and O-rich environments, respectively. The Bi- and BiO-terminations of single-layer BiOX are found to have obviously active sites for HER, whereas the (001) basal planes are inert. The Gibbs free energies for the adsorption of hydrogen atoms on the Bi- and BiO-terminations are close to the optimal value of 0 eV, indicating that single-layer BiOX possess favorable HER performances. The enhanced HER activities on the Bi- and BiO-terminations are attributed to the localized edge states around the Fermi level, which are caused by the Bi 6p-orbital density of the fringe bismuth atoms and O 2p-orbital density of the fringe oxygen atoms, respectively. The results of this work suggest that single-layer BiOX are a family of promising catalysts for water splitting.