Geologic records of Earth's hydrosphere and meteoric precipitation older than 2 Ga are rare, although they provide insight into the past climate, rates of water-rock interaction, and intensity of plate tectonics. Here we report and describe in detail the lowest known δ18O (–16‰ to –25‰) terrestrial silicate rocks on Earth, found in Paleoproterozoic plagiogneisses from the Belomorian complex, Karelia, Russia. Geochronologic and oxygen isotopic data on zircons (+7‰ to –26‰) and monazite (–17.5‰) imply that the protoliths of these rocks were ca. 2.5 Ga metasediments and metavolcanics that were hydrothermally altered prior to 1.85 Ga within an intracontinental rift zone, and involved ultralow δ18O, <–25‰ meteoric water. Paleogeographic reconstructions indicate that Karelia was at low to middle latitudes throughout the Paleoproterozoic Era. Ultradepleted δ18O waters outside of polar regions or the interiors of large landmasses provide independent evidence for a moderately glaciated, so called “slushball” Earth climate between 2.45 and 2.4 Ga, in which low- or mid-latitude, mid-size continents were covered with glaciers while the ocean remained at least partially unfrozen to allow for intracontinental isotopic distillation in a large temperature gradient. In addition to these climatic inferences, the data are more readily explained by a depleted –10‰ seawater reservoir during Paleoproterozoic time.