Prison officers’ behavior is one of the most consequential features of the modern prison. In this article, we introduce an organizational culture conceptual framework and build on previous prison scholarship to develop a model of prison officer workplace culture. We then apply the proposed model to original research in a US prison to investigate the relational aspects of prison officer culture during early 21st-century penal reforms. We find a set of collective norms and beliefs among officers consistent with the “traditional” prison officer culture historically documented by penologists, including high levels of distrust of prisoners, avoidance of relationships, and distancing from rehabilitation goals. We name this culture the “security mindset” because officers use multiple conceptions of “security” to rationalize their behavior. Our findings suggest that prison officer culture in late mass incarceration may work against the positive and supportive relationships necessary for rehabilitation.