Immigration scholars have increasingly questioned the idea that “illegality” is a fixed, inherent condition. Instead, the new consensus is that immigration laws produce “illegality.” But can “illegality” be socially constructed? When initially judging who is an “illegal immigrant,” common observers and even authorities typically do not rely on an individual’s documentation. Instead, people rely on shared stereotypes to assign “illegality” to certain bodies, a condition we refer to as “social illegality.” Ethnographers have documented that individual traits like occupation or national-origin may trigger illegality suspicions, but it is not clear how widespread these stereotypes are, or whether all stereotypes are equally consequential. To address this question, we examine the personal attributes shaping perceived “illegality.” We apply a paired conjoint survey experiment on a nationally representative sample of 1,515 non-Hispanic white U.S. adults to assess the independent effect of each dimension. We find that national origin, social class, and criminal background powerfully shape perceptions of illegality. These findings reveal a new source of ethnic-based inequalities—“social illegality”—that may potentially increase law enforcement scrutiny and influence the decisions of hiring managers, landlords, teachers, and other members of the public.