Segregation, particularly on the basis of race, is widely understood in the United States to be problematic, deleterious to equality, and apt to reproduce conditions of domination by the privileged group over the marginalized minority. The widespread practice of segregated spaces has been largely outmoded and legally banned in most U.S. policy domains, except in the case of sports. Not only do American high schools and universities consistently promote one team for girls and another for boys, the dominant policy regime designed to end sex discrimination in educational settings—Title IX of the Educational Amendments of 1972—promotes this use of segregation “on the basis of sex” as the non-discriminatory policy solution. This paper, based on archival evidence from U.S. federal government and presidential archives, traces the roots of sex segregation from Title IX’s inception to the modern day. It argues that this segregatory practice, wherein girls and boys are kept apart on competitive teams “by virtue of sex”, is less benign than it may appear. Segregation is the opposite policy solution to that pursued in classroom spaces, where sex integration has been almost unilaterally pursued in the U.S. public education context. Sex segregation in sports is so naturalized that it often appears “hidden in plain sight”—both in educational institutions regulated by Title IX and in international sports competitions, such as the Olympic Games. However, the widespread practice of sex segregation requires an historicized re-thinking. Turning to history and the extant scholarship on segregatory policy practices more generally, demonstrates that the curious practice of segregation through Title IX casts this element of U.S. civil rights policy as an extraordinary outlier from its policy cognates, both within the U.S. and in comparative perspective. Segregation, in this light, is as much “problem” as gendered policy “solution” to sex discrimination in athletics.