The early volcanic history and structural development of Olympus Mons, Mars, has been obscured by the formation of the large circum-Olympus aureole deposits in the Early Amazonian. Mars Orbiter Laser Altimeter data reveal an enormous preaureole extrusive flow unit in Amazonis Planitia that is more than 300 km wide and extends ∼1800 km radially away from the Olympus Mons aureole. We interpret this volcanic unit to represent proto–Olympus Mons lava flows emplaced during the latest Hesperian or earliest Amazonian. The orientation and localization of the deposits appear to be due to channeling of the flows into a broad depression between the generally north dipping slope of Amazonis Planitia and the southwestward-dipping slope of the Alba Patera flanks. Formation of the aureole blocked this depression and caused subsequent summit flows to pond in the circum- Olympus trough and flow around the aureole lobes. Emplacement of this proto- Olympus flow unit formed a barrier along the northern margin of Amazonis Planitia, causing ponding of later lava flows and outflow channel events debouching there. The large volume and great lateral extent of the unit imply abundant magma supply and high effusion rates during the initial stages of Olympus Mons construction; this unit may be the result of the initial impingement of a melt-rich mantle plume head.