In order to keep up with the pace of innovation, military firms have recently launched a series of open innovation (OI) initiatives to search for and integrate external knowledge into their internal development process. Adopting OI in such a secretive environment unlocks new possibilities to analyze how firms can pursue openness and secrecy. This article builds on a qualitative research conducted inside a large military firm that has implemented an inbound OI strategy. Relying on multiple case studies and interviews with individual players involved in the firm's OI initiatives, we analyzed how these players deploy secrecy practices when participating to OI projects. They actually combine cognitive practices (aiming at modulating the contextual depth of the knowledge revealed through reframing) with relational practices (aiming at controlling the visibility and exposure of this knowledge). We highlight how these combinations evolve during the lifecycle of OI partnerships. By emphasizing different modes by which individual actors practice secrecy in OI, we contribute to previous research addressing how organizations navigate the paradox of openness. Besides, this study proposes new theoretical insights on the role and features of secrecy practices in innovation activities, and thus contributes to the emerging research field of managerial secrecy. •Secrecy in open innovation is managed through cognitive and relational practices.•Cognitive practices are about moderating contextual depth of knowledge.•Relational practices are about moderating the visibility breadth of knowledge.•OI players combine secrecy practices to navigate the paradox of openness.