This dissertation examines Charles S. Peirce's writings on Plato. Peirce's lifelong reading of Plato, and especially of Plato's late dialogues in the 1890s, was influential in the development of Peirce's Pragmatism. Peirce claimed that Plato misunderstood himself and was, late in his career, developing an evolutionary, three-level metaphysics and logic that anticipated Peirce's Pragmaticism.The first half of the dissertation deals with Peirce and the history of philosophy. Chapters address the case for this study, Peirce's method of studying the history of philosophy and his use of intellectual autobiography, and Peirce's encounter with Platonism in America. A running theme throughout the dissertation is Peirce's engagement with mysticism and American Transcendentalism, and especially with Emerson, Thoreau, and Henry James, Sr. Also discussed here is Peirce's reception of stylometric analyses of the Platonic dialogues through Wincenty Lutoslawski's Origin and Growth of Plato's Logic . The section on stylometrics also briefly discusses Lewis Campbell and contemporary views on stylometrics in Brandwood, Thesleff, and Nails.The second half of the dissertation examines Peirce's writings on particular Platonic dialogues and the consequences of those writings for Peirce's Pragmaticism. Chapters cover the development of Peirce's metaphysics, his ethics of inquiry, etymology and the ethics of terminology, and miracles and their relation to scientific and historical research. The chapter on miracles also examines the role Plato played in Peirce's response to Hume on miracles.Individual chapters address Peirce's writings on Plato's Cratylus and Theaetetus. The Apology, Parmenides , and Sophist are also discussed. Especial attention is paid to Peirce's unpublished manuscripts, as well as to his entries on "Platonic" and "Socratic" in the Century Dictionary, his own account of his debt to Aristotle, his intellectual autobiographies, and his manuscripts entitled "Metaphysical Axioms and Syllogisms," "A Neglected Argument for the Reality of God," "Philosophy and the Conduct of Life," "On the Logic of Drawing History from Ancient Documents," and "The Law of Mind."Appendices include transcripts of Peirce's unpublished partial translations of Plato's Cratylus; a transcript of an unpublished letter from Peirce to Lady Welby on the Theaetetus; and a catalog of references to Plato in Peirce's manuscripts.