Health care workers are often exposed to violence and aggression in psychiatric settings. Short-term risk assessments, such as the Brøset Violence Checklist (BVC), are strong predictors of such aggression and may enable staff to take preventive measures against aggression. This study evaluated whether the routine use of the BVC could reduce the frequency of patient aggression. We conducted a study with a semi-random regression discontinuity design in 15 psychiatric wards. Baseline aggression risk was assessed using the Aggression Observation Short Form (AOS) over three months. The BVC was implemented in seven intervention wards, and the risk of aggressive incidents over three months of follow-up was compared with the risk in eight control wards. The analysis was conducted at the ward level because each ward was allocated to the intervention and control groups. At baseline, the risk of aggression varied between wards, from one aggressive incident per patient per 1,000 shifts to 147 aggressive incidents per patient per 1,000 shifts. The regression discontinuity analysis found a 45% reduction in the risk of aggression (Odds Ratio (OR) = 0.55, 95% confidence interval: 0.21–1.43). The study did not find a significant reduction in the risk of aggression after implementing a systematic short-term risk assessment with the BVC. Although our findings suggest that use of the BVC may reduce the risk of aggression, the results need to be confirmed in studies with more statistical power.