The incarceration of transgender prisoners in men’s prisons is a burgeoning topic of legal challenge, policy development, and social science inquiry. The conspicuous absence of comparable attention to women’s facilities may facilitate a tacit assumption that what is known about transgender prisoners in men’s prisons translates seamlessly to women’s facilities. This paper interrogates this assumption by examining current understandings of what it means to be transgender in a women’s prison. Findings from focus groups with prisoners and staff reveal that gender is understood as both reflected by and constituted through social interaction. Specifically, in attempting to explain the concept of “transgender” in women’s prisons, this work instead reveals a different prevailing concept in prisoner culture: “aggressor.” Unlike transgender, aggressor does not denote gender identity; rather, it implies presentation and performance as reflective of gendered ways of navigating relationships within the context of a sex-segregated setting. These findings simultaneously affirm the extant literature on gender and sexuality in women’s prisons and complicate the translation of the identity-based concept “transgender” from men’s prisons to a women’s prison.