Ensuring effective mental health and psychosocial support is crucial following exposure to a potentially traumatic event and can have long-term consequences for individuals, families, and communities. Psychological first aid (PFA) has become a widespread intervention of choice following exposure to conflict or disaster; however, its impact is unknown. This systematic review assessed PFA efficacy in improving the mental health and psychosocial well-being of individuals exposed to potentially traumatic events. We searched PubMed, PsycINFO, PTSDpubs, and EMBASE for peer reviewed studies evaluating programmatic outcomes of PFA, or an adapted intervention, published in English before March 9, 2021. Studies evaluating training outcomes or program feasibility were excluded. The primary outcomes were reported measures of participant mental health and psychosocial well-being, with narrative results presented for each. The Cochrane Risk of Bias tool was applied. Of 9,048 potentially eligible citations, 12 studies with a total of 1,437 participants met the inclusion criteria. Only one study was a randomized controlled trial. The findings from all studies suggest a positive impact of PFA, with most reporting reduced symptoms of anxiety, depression, posttraumatic stress, and distress, as well as improved ratings of mood, the experience of safety, connectedness, and a sense of control, among youth and adults. Risk of bias was generally high. Inconsistent intervention components, insufficient evaluation methodologies, and a high risk of bias within the reviewed studies present challenges in assessing PFA efficacy, and an imbalance between popular support for PFA and scant evidence of outcome data exists.