Combined archaeological and ecological investigations in a large ancient oak forest in Central France have revealed a dense network of ancient human settlements dating from the Roman period. We demonstrate a strong correlation between present‐day forest plant diversity patterns and the location of Roman farm buildings. Plant species richness strongly increases toward the center of the settlements, and the frequency of neutrophilous and nitrogen‐demanding species is higher. This pattern is paralleled by an increase in soil pH, available P, and δ¹⁵N, indicating the long‐term impact of former agricultural practices on forest biogeochemical cycles. These extensive observations in a forested region on acid soils complement and confirm previous results from a single Roman settlement on limestone. Ancient Roman agricultural systems are increasingly being identified in contemporary French forests; the broad extent and long‐lasting effects of previous cultivation shown in this study require that land‐use history be considered as a primary control over biodiversity variations in many forest landscapes, even after millennia of abandonment.