Psychological first aid (PFA) is a world-wide implemented approach to helping people affected by an emergency, disaster, or other adverse event. Controlled evaluations of PFA’s training effects are lacking. We evaluated the effectiveness of a one-day PFA training on the acquisition and retention of knowledge of appropriate responses and skills in the acute aftermath of adversity in Peripheral Health Units (PHUs) in post-Ebola Sierra Leone. Secondary outcomes were professional quality of life, confidence in supporting a distressed person, and professional attitude. PHUs in Sierra Leone (n = 129) were cluster-randomized across PFA (206 participants) and control (202 participants) in March 2017. Knowledge and understanding of psychosocial support principles and skills were measured with a questionnaire and two patient scenarios to which participants described helpful responses. Professional attitude, confidence, and professional quality of life were assessed using self-report instruments. Assessments took place at baseline and at three- and six-months post-baseline. The PFA group had a stronger increase in PFA knowledge and understanding at the post-PFA training assessment (d = 0.50; p < 0.001) and at follow-up (d = 0.43; p = 0.001). In addition, the PFA group showed better responses to the scenarios at six-months follow-up (d = 0.38; p = 0.0002) but not at the post-assessment (d = 0.04; p = 0.26). No overall significant differences were found for professional attitude, confidence, and professional quality of life. In conclusion, PFA training improved acquisition and retention of knowledge and understanding of appropriate psychosocial responses and skills in providing support to individuals exposed to acute adversity. Our data support the use of PFA trainings to strengthen capacity for psychosocial support in contexts of disaster and humanitarian crisis. Future studies should examine the effects of PFA on psychosocial outcomes for people affected by crises.