A major debate in Mars science is the nature of the early Mars climate, and the availability of precipitation and runoff. Observations of relict erosional valley networks have been proposed as evidence for extensive surface runoff around the Noachian-Hesperian boundary. However, these valley networks only provide a time-integrated record of landscape evolution, and thus, the timing, relative timescales and intensity of aqueous activity required to erode the valleys remain unknown. Here, we investigate an ancient fluvial sedimentary system in western Arabia Terra, now preserved in positive relief. This ridge, “Aram Dorsum,” is flat-topped, branching, ~85 km long, and particularly well preserved. We show that Aram Dorsum was an aggradational alluvial system and that the existing ridge was once a large river channel belt set in extensive flood plains, many of which are still preserved. Smaller, palaeochannel belts feed the main system; their setting and network pattern suggest a distributed source of water. The alluvial succession is up to 60 m thick, suggesting a formation time of 105 to 107 years by analogy to Earth. Our observations are consistent with Aram Dorsum having formed by long-lived flows of water, sourced both locally, and regionally as part of a wider alluvial system in Arabia Terra. This suggests frequent or seasonal precipitation as the source of water. Correlating our observations with previous regional-scale mapping shows that Aram Dorsum formed in the mid-Noachian. Aram Dorsum is one of the oldest fluvial systems described on Mars and indicates climatic conditions that sustained surface river flows on early Mars.