Haploidoceros mediterraneus is one of the recently described cervid taxa endemic to the Iberian Peninsula (three sites) and southern France (two sites). Compared to the other endemic cervids from Iberia that have emerged, as well, during the mid-Middle Pleistocene, its chronological and geographical range are more expanded, indicative of a relative adaptative success. However, very little is known about H. mediterraneus ecology. From site contexts and faunal associations, its habitat during the Middle and Late Pleistocene corresponded to open forest under a mildly-humid temperate Mediterranean or semi-continental climate. First, its diet was reconstructed using dental meso-and microwear and second, its relationships with other cervids over time was analysed using Multiple Component Analysis (MCA). Diet reconstruction indicates that it was mostly a browse-dominated mixed-feeder during the Middle Pleistocene. Such a relatively flexible diet has allowed it to coexist, often with some cervid taxa (Cervus and Capreolus) and exceptionally with others (Praedama/Megaloceros and Dama sp.) by resource partitioning. During the Late Pleistocene, H. mediterraneus shifted toward a more browse-specialized diet. At this period, co-occurrence analyses show that the presence of H. mediterraneus seems to exclude that of C. capreolus and D. dama. The evolution of H. mediterraneus dietary habits from the Middle to the Late Pleistocene may have led to direct competition with other browsing-specialist cervids, which may be one of the causes of its extinction.