While transitional justice endeavors aim to help countries come to terms with violent pasts, policymakers and practitioners often claim that transitional justice mechanisms help prevent future violence as well. No cross-national research has tested this claim, however. This article begins to fill this gap by examining whether one of the most frequently used mechanisms of transitional justice, the truth commission, is associated with the onset of targeted mass killings (TMKs) in 27 countries between 1972 and 2018. Our differences-in-differences estimation approach finds that countries that have implemented truth commissions see a significant reduction in the recurrence of TMKs compared to those countries that did not implement a truth commission. Additional analyses reveal that while issuance of a final report and recommendations for reforms are not associated with the onset of TMKs, truth commissions that do not recommend the punishment of perpetrators are more likely to reduce TMKs in the years following the truth commission. The article ends by discussing the functions of truth commissions and proposing how their preventive capacity may be strengthened further through the application of an atrocity prevention lens.