Mars shergottite hydrogen isotope ground ice hydrated crust ion microprobe Fluvial landforms on Mars suggest that it was once warm enough to maintain persistent liquid water on its surface. The transition to the present cold and dry Mars is closely linked to the history of surface water, yet the evolution of surficial water is poorly constrained. Based on in situ hydrogen isotope (D/H) analyses of quenched and impact glasses in Martian meteorites, we provide evidence for the existence of a distinct but ubiquitous water/ice reservoir (D/H = ∼2–3 times Earth's ocean water) that lasted from at least the time when the meteorites crystallized (173–472 million years ago) to the time they were ejected by impacts (0.7–3.3 million years ago), but possibly much longer. The origin of this reservoir appears to predate the current Martian atmospheric water (D/H = ∼5–6 times Earth's ocean water) and is unlikely to be a simple mixture of atmospheric and primordial water retained in the Martian mantle (D/H ≈ Earth's ocean water). This reservoir could represent hydrated crust and/or ground ice interbedded within sediments. Our results corroborate the hypothesis that a buried cryosphere accounts for a large part of the initial water budget of Mars.