Objective: In forensic psychiatric patients, sleep problems as well as impulsivity and aggression are highly prevalent, yet studies on their association over time are lacking. This study investigates the association between sleep quality and changes in impulsivity and aggression in forensic psychiatric patients over one year. Methods: Data were drawn from an ongoing prospective observational study in adult forensic psychiatric patients admitted to a forensic treatment facility between October 2006 and January 2018. Validated self-reports and observational instruments were used to assess sleep quality, impulsivity and aggression upon admission to the hospital and after one year. Linear regression analyses were performed to examine the association between sleep quality, impulsivity and aggression. All models were adjusted for baseline values of outcome measures, demographic features and general psychopathology. Results: Data from 83 men (age 37.7 ± 11.7 years) with completed consecutive measurements were analyzed. Poor sleep quality was associated with increased self-reported aggression (β = 1.08; 95% CI, 0.38–1.78). This association was positively confounded by general psychopathology, indicating that sleep quality is specifically related to self-reported aggression instead of being part of general psychopathology (adjusted β = 1.18; 95% CI, 0.39–1.97). Poor sleep quality was not associated with changes in self-reported impulsivity, clinician-rated impulsivity or clinician-rated hostility in this population. Conclusion: Poor sleep quality was associated with an increase in self-reported aggression over one year in male forensic psychiatric patients. Early evaluation and treatment of sleep problems in (forensic) psychiatric patients may play an important role in reducing the risk of aggressive behavior.