The goal of this article is to contribute to ongoing efforts to strengthen gender regimes theory by proposing a new sub domain within the institutional domain of violence that brings in the penal state – any part of formal politico-administrative structures that arrests and incarcerates women. Based on interdisciplinary research on gender, crime and punishment that shows the importance of understanding the penal state for women's rights and gender equality, the operational definition of the penality sub domain is proposed and developed in a case study of prison reform and gender equality policy in Spain. The concept development case study not only helps to define and empirically identify the seven dimensions of the new sub domain. It also provides the opportunity to make a larger theoretical reflection of what penality issues mean for the application of gender regimes theory more broadly speaking and future comparative studies of gender equality, prisons and gender regimes.