Jefferson Allen McMahan (born 30 August 1954) is an American philosopher. He is currently White's Professor of Moral Philosophy at the University of Oxford.

McMahan has published extensively on topics of interest to effective altruism, such as population ethics (McMahan 1981; McMahan 2013); existential risk from nuclear war (McMahan 1986); animal ethics—including the ethics of eating animals (McMahan 2008), the moral status of animals (McMahan 2005), and wild animal welfare (McMahan 2010a; McMahan 2010b; McMahan 2015; cf. Faria 2015)—; and the demandingness of morality (McMahan 2018). He has also published a response to various criticisms of effective altruism (McMahan 2016).