"Bloomsbury's Byzantium and the Writing of Modern Art" examines the role of Byzantine art in Bloomsbury art critics Roger Fry's and Clive Bell's narratives of aesthetic Modernism. Fry, in his pre-World War I and interwar writings and teachings on art, and Bell, in seminal texts such as Art (1914), have been branded by art historiography as the prime movers in a Formalist, teleological narrative of Modern art still prevalent in textbooks today. Fry's and Bell's ideas were later adopted by important Modernist authors and cultural figures, such as Alfred H. Barr, Jr., first director of New York's Museum of Modern Art, and critic Clement Greenberg. Yet, less known is the integral role Byzantine art played in delimiting both Fry's and Bell's ideas of Modernism, and the art works they valued. Consistent with the international nineteenth- and twentieth-century interest in Byzantium, Fry and Bell each crafted an ahistorical idea of Byzantium. The Bloomsbury critics' highly subjective definitions of Byzantine art and the Byzantine era allowed both Fry and Bell to project onto Byzantium qualities that aligned with their own intellectual interests. My dissertation uses these varied characterizations of Byzantium to reinterpret both authors' writings on Modern art and subsequently to challenge canonical understanding of Western aesthetic Modernism. For instance, in my analysis of Fry's and Bell's idea of Byzantine art, I point to parallel qualities the critics' valued in Modern pieces; and suggest that they used their concept of Byzantium to define a more secular, universalized spiritual conception of art as an alternative and counterpart to mainstream religions. I also explain how the critics relied on their definition of Byzantium to each advocate for non-Western art's aesthetic value, and I demonstrate how the authors utilized their characterization of Byzantine art to contest the precedent of both John Ruskin and establishment, Western art history. This dissertation unravels the myriad personal, intellectual, and contextual circumstances which led to Fry's and Bell's interpretation of Byzantine art, and, as a result, illustrates how art-world politics and world politics of the late-nineteenth and early-twentieth centuries impacted the writing of formative texts in Western Modern art.