Hydrogen is a high energy content fuel that can be produced with low or zero greenhouse gas emissions from water and other chemicals. Creating hydrogen during periods of energy surplus and storing it underground is one long-duration, low-emission, energy storage option that can balance supply and demand for an entire electric grid. In the United States (U.S.), existing underground gas storage (UGS) facilities are a logical first place to consider subsurface hydrogen storage, because their geology has proven favorable for storing natural gas. We estimated that existing UGS facilities can store 327 TW-h (9.8 million metric tons) of pure hydrogen. Transitioning from natural gas to pure hydrogen storage would reduce the total energy stored in existing UGS facilities by ∼75%. Storing hydrogen-natural gas mixtures also reduces energy storage potential, but most (73.2%) UGS facilities can meet current energy demands with a 20% hydrogen blend. U.S. UGS facilities can store 23.9%-44.6% of the projected high and low hydrogen demand for 2050, respectively, suggesting that a partial transition of UGS infrastructure could reduce the need for new hydrogen storage facilities. These findings motivate research that explores the technical feasibility of underground hydrogen storage in natural gas storage reservoirs. LACKEY ET AL.