Electronic structure calculations based on density functional theory are used to identify the catalytically active sites for the hydrogen evolution reaction on single layers of the two transition metal tri-chalcogenide compounds CoPS3 and NiPS3. Some of the under-coordinated P and S atoms at the edges are found to act as the active sites, the details of which depend on the coverage of H on the electrode. Overpotentials along the two possible pathways for HER are also estimated for the two materials. These findings not only resolve an apparent discrepancy between published experimental results and our earlier calculations, but also provide insights which can be used to enhance catalytic efficiency of these materials further.