The origins of olivine-rich rocks on Mars remain only partially constrained, with hypothesized deposition as lava flows, impact products, aeolian sand, and pyroclastic materials. These rock units’ uncertain origins obscure their genetic relationships with each other and the contexts of their aqueous alteration. We synthesize geomorphic mapping and geomorphic and stratigraphic measurements with previous spectroscopic analyses to constrain the origin of the circum–Isidis Planitia olivine-rich unit, one of Mars’s most widespread and mineralogically diverse olivine-rich rocks. We find that the unit most likely formed as an ash-fall deposit, with a probable origin related to volcanism in the greater Syrtis Major–Isidis Planitia region. This work corroborates hypotheses that some extensive outcrops of ancient bedrock are clastic and that a planet-wide transition from dominantly explosive to effusive volcanism may have occurred in the Hesperian. Our findings also highlight the likely diverse origins of olivine-rich martian rocks and provide key geologic context for the aqueous alteration of the unit and underlying ancient crust.