Study of the hydrothermal dynamics of Yellowstone Lake (Wyoming, USA) is important for identifying potential changes in sublacustrine hydrothermal systems in response to external perturbations from earthquakes, seiches, large waves, and seasonal effects. Remotely operated vehicle (ROV) submersible-based investigations of hydrothermal vents offshore from Stevenson Island reveal numerous non-constructional ~10-cm-diameter orifices with diffuse fluid flow at temperatures up to 174 °C. The vent field occurs in a large roughly conical depression on the lake floor at a water depth of ~120 m. The volatile-rich composition (CO2, H2S) of the vent fluids is preserved by using a novel isobaric sampling system that precludes degassing effects. In addition to high temperatures, the vent fluids have high CO2 and H2S, but low chloride (Cl) and major element concentrations largely indistinguishable from those in ambient lake water. These results are consistent with steam addition to the sublacustrine hydrothermal system. Kaolinite- and boehmite-rich alteration indicates acidic conditions and provides a low-permeability substrate that may contribute to the development of a steam-heated upflow zone. At the scale of individual vent areas (centimeters to meters), perturbations cause bursts of steam-rich fluids that locally expel and disperse sediment and contribute to the formation of vent orifices. Here we report on chemical and physical phenomena associated with the hottest and deepest sublacustrine hydrothermal vents in Yellowstone Lake. Results indicate that vapor-dominated sublacustrine systems are fundamentally different in hydrothermal alteration and hydrothermal dynamic characteristics than their liquid-dominated counterparts.