Human behavior is not always consistent with standard rational choice predictions. Apparent deviations from rational choice predictions provide a promising arena for the merger of economics and biology [1-6]. Although little is known about the extent to which other species exhibit these seemingly irrational patterns [7-9], similarities across species would suggest a common evolutionary root to the phenomena. The present study investigated whether chimpanzees exhibit an endowment effect, a seemingly paradoxical behavior in which humans tend to value a good they have just come to possess more than they would have only a moment before [10-13]. We show the first evidence that chimpanzees do exhibit an endowment effect, by favoring items they just received more than their preferred items that could be acquired through exchange. Moreover, the effect is stronger for food than for less evolutionarily salient objects, perhaps because of historically greater risks associated with keeping a valuable item versus attempting to exchange it for another [14, 15]. These findings suggest that many seeming deviations from rational choice predictions may be common to humans and chimpanzees and that the evaluation of these through a lens of evolutionary relevance may yield further insights in humans and other species.