High-resolution topographic data from the Mars Orbiter laser altimeter reveal evidence for widespread denudation from Margaritifer Sinus to northern Arabia Terra, an area of ∼1 × 107 km2. A major resurfacing event is indicated by: (1) a heavily degraded landscape hosting numerous inliers, (2) truncation or absence of valley networks, and (3) a break in slope at the edge of early to middle Noachian plateau materials. Geomorphic mapping was completed on the type locale of denudation (0°–30°S, 0°–30°W). Superposition relations and crater counts for geomorphic units indicate that large-scale resurfacing took place in the late Noachian, eroding the Martian highlands and resulting in transportation and deposition of ∼4.5 × 106 km3 of sediment in the northern plains. This is equivalent to a 120-m-thick uniform layer of sediment on the surface of Mars north of 30°N. Geomorphic mapping and crater counts limit the timing of denudation to the late Noachian, an interval of 350–500 m.y. Using this limit, we estimate a minimum rate of denudation of 2.0 μm/yr, comparable to denudation of typical slopes in a temperate maritime climate on Earth. The morphology, rate of denudation, and extensive nature of upland degradation suggest that precipitation-fed surface runoff is the most likely geomorphic agent capable of such a process, indicative of a warm, wet Mars during the late Noachian Epoch.