This dissertation documents the rise of what I term the “sexual carceral state,” a network of laws, civil regulations, and institutions developed in the twentieth-century United States to punish and control people considered sexually deviant and dangerous. Most prominently, in the midcentury, states across the country passed sexual psychopath laws—also called psychopathic personality, sexually dangerous person, and mentally disordered sex offender laws—which allowed for the indefinite civil detention of people deemed mentally abnormal and dangerous. Although the sexual psychopath laws adopted the language of medicine and treatment, they were, I argue, a means to circumvent constitutional rights granted to defendants in prosecutions and statutory limits on sentences. They enabled the indefinite preventive detention of people accused of serious sexual violence but also of queer people who engaged in consensual sex, Black men who crossed racial boundaries, and people accused of minor transgressions. This dissertation shows that the construction of the sexual carceral state began in the 1910s and 1920s, decades earlier than has been established in existing scholarship, as the public grew alarmed at newspaper coverage of sex crimes committed by so-called “morons”—people who were thought to suffer from intellectual, affective, or moral defects. These panics drew attention to male sexual violence and sexual variation, which authorities sought new ways to control. During this period, psychopathic laboratories looked for deviants hidden among the populations passing through courts, police departments, and prisons while police departments in major cities developed lists of suspected deviants who might be dangerous. Starting in the 1930s, states across the country passed sexual psychopath laws. Finally, a few decades later, some states developed special institutions to confine people accused of sex offenses. This dissertation also reveals how the people targeted by the sexual carceral state contested its rise—particularly the sexual psychopath laws. It explores previously unexamined evidence, including case records, correspondence between lawyers and people detained as sexual psychopaths, and detainees’ writings. I show that by the 1970s, litigation brought by people detained under sexual psychopath laws had established rights to greater procedural protections in psychopath proceedings, to treatment, and against detention in prisons. These changes, I argue, made the sexual psychopath laws less useful to prosecutors, more expensive for states, and less attractive to those who wanted a more punitive approach to sex crime. As a result, throughout the 1970s and 1980s, many states repealed their sexual psychopath laws or stopped using them, which led indirectly to the passage of new sex offender regulations in the 1990s. Despite the complicated nature of its legacy, the litigation against sexual psychopath laws forced courts to begin to regulate mental hospitals, brought attention to psychiatric abuse and neglect, and weakened a tool that states had used to regulate queer sexuality. This dissertation offers new insights into the origins of sex offender legislation, prisoner and patient rights activism, the relationship between law and psychiatry, the regulation of sexuality, and the criminal legal system in the twentieth-century United States.