Background: Despite its popularity, evidence of the effectiveness of Psychological First Aid (PFA) is scarce.Objective: To assess whether PFA, compared to psychoeducation (PsyEd), an attention placebo control, reduces PTSD and depressive symptoms three months post-intervention.Methods: In two emergency departments, 166 recent-trauma adult survivors were randomised to a single session of PFA (n = 78) (active listening, breathing retraining, categorisation of needs, assisted referral to social networks, and PsyEd) or stand-alone PsyEd (n = 88). PTSD and depressive symptoms were assessed at baseline (T0), one (T1), and three months post-intervention (T2) with the PTSD Checklist (PCL-C at T0 and PCL-S at T1/T2) and the Beck Depression Inventory-II (BDI-II). Self-reported side effects, post-trauma increased alcohol/substance consumption and interpersonal conflicts, and use of psychotropics, psychotherapy, sick leave, and complementary/alternative medicine were also explored.Results: 86 participants (51.81% of those randomised) dropped out at T2. A significant proportion of participants in the PsyEd group also received PFA components (i.e. contamination). From T0 to T2, we did not find a significant advantage of PFA in reducing PTSD (p = .148) or depressive symptoms (p = .201). However, we found a significant dose-response effect between the number of delivered components, session duration, and PTSD symptom reduction. No significant difference in self-reported adverse effects was found. At T2, a smaller proportion of participants assigned to PFA reported increased consumption of alcohol/substances (OR = 0.09, p = .003), interpersonal conflicts (OR = 0.27, p = .014), and having used psychotropics (OR = 0.23, p = .013) or sick leave (OR = 0.11, p = .047).Conclusions: Three months post-intervention, we did not find evidence that PFA outperforms PsyEd in reducing PTSD or depressive symptoms. Contamination may have affected our results. PFA, nonetheless, appears to be promising in modifying some post-trauma behaviours. Further research is needed.