There are thousands of fossils of hominins, but no fossil chimpanzee has yet been reported. The chimpanzee (Pan) is the closest living relative to humans. Chimpanzee populations today are confined to wooded West and central Africa, whereas most hominin fossil sites occur in the semi-arid East African Rift Valley. This situation has fuelled speculation regarding causes for the divergence of the human and chimpanzee lineages five to eight million years ago. Some investigators have invoked a shift from wooded to savannah vegetation in East Africa, driven by climate change, to explain the apparent separation between chimpanzee and human ancestral populations and the origin of the unique hominin locomotor adaptation, bipedalism. The Rift Valley itself functions as an obstacle to chimpanzee occupation in some scenarios. Here we report the first fossil chimpanzee. These fossils, from the Kapthurin Formation, Kenya, show that representatives of Pan were present in the East African Rift Valley during the Middle Pleistocene, where they were contemporary with an extinct species of Homo. Habitats suitable for both hominins and chimpanzees were clearly present there during this period, and the Rift Valley did not present an impenetrable barrier to chimpanzee occupation.