This project is a comparative textual study of the ontologies and ethics of Jean-Paul Sartre and Nishitani Keiji in order to establish a basis for a socially engaged Zen Buddhist ethics as exemplified in the Engaged Buddhism movement. Initially, I develop a Sartrean ethics of fraternity based on a movement from independent self-centered personal freedom to an interdependent universal freedom. However, shortcomings of this Sartrean ethics are illustrated through examining Nishitani Keiji's Zen Buddhist philosophy of emptiness. Finally, Mahayana Buddhist philosophers including Vimalakirti, Nagarjuna, and Dogen are brought out to develop a socially engaged Buddhism which has become actualized in today's Engaged Buddhism movement.