Background: An understanding of the underlying processes that promote or prevent treatment engagement is limited in forensic populations despite treatment being implicated in improved outcomes. Attachment and relatedness have been linked to motivational processes and treatment engagement. Increased attachment insecurity is thought to reduce treatment engagement, whereas increased perceptions of relatedness is linked to increased motivation. Aims: To explore how attachment-anxiety and attachment-avoidance and patient’s perceptions of relatedness impact treatment readiness and motivation in secure forensic populations. It was hypothesised that perceptions of therapeutic relatedness would mediate the relationship between attachment and treatment readiness and motivation. Methods: A cross-sectional, mixed methods design was used. Seventy adult forensic patients were recruited from medium and high secure hospitals. Participants were administered a series of self-report questionnaires exploring attachment, relatedness, treatment readiness and motivation. Additionally, participants answered open-ended questions exploring treatment engagement. Results: Relatedness was significantly correlated to treatment readiness but not patient motivation. There was a small but non-significant negative correlation between attachment-avoidance and treatment readiness and a small but non-significant positive correlation between attachment-anxiety and patient motivation. There was a small significant negative indirect effect of attachment-avoidance on treatment readiness through perceptions of relatedness. Although as confidence intervals were close to zero this is tenuously concluded. All other mediation models were non-significant. Open-ended questions produced three main categories exploring motivators and barriers to treatment and perceptions of relationship quality with clinicians. Conclusions: As attachment and relatedness were found to be related to treatment readiness and patient motivation, assessing these factors could support the development of care plans which target increasing engagement. The findings provide some support for treatment readiness and motivation theories, however, the inconclusive results related to the indirect pathways indicate that other factors may be relevant that require further exploration.