During CO2 electroreduction (CO2R), the hydrogen evolution reaction (HER) is a competing reaction. We present a combined experimental and theoretical investigation of the HER activity of transition metals under CO2R conditions. Experimental HER polarization curves were measured for six polycrystalline metal surfaces (Au, Ag, Cu, Ni, Pt, and Fe) in the presence of CO2 gas. We found that the HER activity of the transition metals is significantly shifted, relative to the CO2-free case. Density functional theory (DFT) calculations suggest that this shift arises from adsorbate–adsorbate interactions between *CO and *H on intermediate and strong binding metals, which weakens the *H binding energy. Using a simple model for the effect of *CO on the *H binding energy, we construct an activity volcano for HER in the presence of CO2 gas that is consistent with experimental trends. The significant changes in HER activity in the presence of CO2 gas is an important consideration in catalyst design and could help develop catalysts that are more selective for CO2R than the HER.