The revelation that the Trump administration separated immigrant children from their families at the U.S.–Mexico border and placed them in detention facilities sparked protests across the country in 2018. While the policy received swift backlash from the public and was widely derided as running counter to American values and the rule of law, a segment of the American public supports the policy. We argue that ethnocultural forms of nationalism—beliefs about religious, ethnic, and gendered criteria for “true Americanness”—help explain support for family separations. We test this argument using two surveys collected 2 years apart. In both data sets, we find substantial evidence that ethnocultural forms of nationalism are linked to support for family separation, while generalized nationalism is not.