Courts and commentators have long assumed, without significant analysis, that immigration detention is a form of civil confinement merely because the immigration proceedings of which it is part are deemed civil. This Article challenges that deeply held assumption. It harnesses the U.S. Supreme Court’s instruction that detention’s civil or penal character turns on legislative intent and, buttressed by theoretical understandings of punishment, contends that immigration detention—apart from the deportation that often results—itself constitutes penal incarceration. In particular, legislation enacted over roughly fifteen years in the 1980s and 1990s indicates a palpable desire to wield immigration detention as a tool in fighting the nation’s burgeoning war on drugs by penalizing and stigmatizing criminal behavior. Indeed, the modern immigration detention system has accomplished the U.S. Congress’s punitive goal: Immigration detention is a severely unpleasant experience and immigration detainees are viewed as dangerous. In order to remain true to the Court’s guidance to draw formal boundaries between civil and penal confinement, the current immigration detention regime should be conceptualized as punishment. This Article contends that the constitutional limitations imposed by criminal procedure are ill-equipped to address immigration detention. Instead, policymakers should learn from the nation’s failed experience with mass penal incarceration—and step back from immigration detention’s punitive origins to create a truly civil immigration detention system.