Submarine lava flows occur regularly near hotspots, mid-ocean ridges, and other tectonically driven magmatic regions. The resulting chemical reactions from a cold, oxidizing ocean circulating through hot, reduced rocks create a suite of potential energy sources for chemosynthetic ecosystems in the deep sea. Subaerial eruptions on volcanic islands can also deposit lava on the seafloor episodically in areas that may otherwise be devoid of hydrothermal activity. To explore the potential for such eruptions to support hydrothermal systems and associated microbial life in the deep sea, here we describe the discovery of low-temperature venting fluids formed on the seafloor lava delta from the Kīlauea eruption of the Big Island of Hawaii in 2018. Approximately 14 weeks after lava began entering the ocean and 2 weeks after ocean entry had ceased, we observed microbial mats and venting fluids at 635 m depth less than a kilometer from the coastline, a site hereby referred to as Haunawela. Fluids emitted from Haunawela were < 1 oC above background seawater, yet they contained an active chemolithoautotrophic community of thermophilic and mesophilic iron, sulfur, and hydrogen-oxidizing autotrophic bacteria typically associated with hydrothermal vents. The composition of the venting fluids and geochemical modeling indicate that the cooling lava flow produced sufficient energy to support sulfur and hydrogen-oxidizing bacteria at higher temperatures and iron-oxidizing bacteria at lower temperatures, perhaps successively as the temperature and chemical composition of mixing fluids evolved with time, or spatially within the subseafloor. Diverse marine chemolithoautotrophic communities supported by water-rock reactions during terrestrial volcanic eruptions into the ocean represent a novel seafloor ecosystem that is underexplored and could support life elsewhere in the Solar System.