Seclusion is a restrictive intervention used in forensic mental health care to manage service user risk of harm. It has been associated with harmful effects for service users and consensus is that its use needs to be reduced. Research has identified that factors related to nursing staff influence the use of seclusion. However, to date no research has considered staff decision-making as it relates to the seclusion process, from initiation to termination. The current aim was to address this gap in the literature. Thirteen senior nurses were recruited from a UK-based service which provided care to men and women in low and medium secure wards. Constructivist Grounded Theory was used to analyse semi-structured interviews that were conducted with participants. Analysis revealed a complex model consisting of numerous overlapping categories that contributed to seclusion decision-making. There were four main categories: cognitive, emotional, relational, and organisational, each with numerous sub-categories. Practice could be improved through using structured assessments to inform seclusion decision-making, providing consistent emotional support for staff who implement seclusion, and focusing on relational approaches to care to build effective therapeutic relationships. There were implications for the importance of organisational policy and leadership and incorporating staff-related factors into seclusion recording processes.