Abstract. To access the impact of anthropogenic emissions and land use change on Earth’s climate and biodiversity, studies into the environment and ecology of natural ecosystems during past warm periods are required. The Miocene Climatic Optimum is a key reference period for future global warming scenarios. However, studies uncovering Miocene climate have heavily favoured marine environments, leaving the impact of warming on terrestrial ecosystems understudied. Here, we present a multi-disciplinary study into the chemical composition of fossil Gomphotherium angustidens (Proboscidea, Mammalia) teeth from the Middle Miocene Vb division (~15.9–16.1 Ma) of western Portugal (Chelas Valley, Lisbon, Lusitanian basin) and their sedimentological context. Trace element and stable isotope compositions in these fossil teeth are compared with similar measurements in molars of a taxonomically related modern African elephant (captive Loxodonta africana). Results reveal seasonal-scale variability in trace elements in both fossil and modern proboscidean tooth enamel, which are interpreted as evidence for seasonal changes in diet. Periodic increases in Na, Fe and Si in G. angustidens demonstrate intake of sediment in the diet during fixed times of the year, a behaviour type previously described in modern elephants during dry seasons. In combination with the heavier carbon and oxygen isotopic composition in G. angustidens compared to L. africana, the terrestrial climate in Miocene Portugal appears characterized by seasonally dry periods, which forced geophagy behaviour of these large mammals and likely had significant consequences for the composition of Miocene ecosystems (e.g., food/water availability and potential seasonal range shifts) in southwestern Europe.