In the last decade, growing concerns about radicalized violence have led governments to make important efforts and invest significant sums of money in developing programs to prevent violent extremism (PVE). Despite these efforts, current knowledge regarding best practices in prevention remains disparate, and the effectiveness of practices used at present has not yet been clearly established. This is especially true for tertiary prevention programs, i.e., those that aim to “deradicalize” and/or disengage individuals from extremist groups and reintegrate them into society. To address this knowledge gap, we conducted a systematic review of the literature published up to 2019 to identify “what works” in tertiary PVE programming. Of the 11,836 studies generated from the searches in this review, 17 were eligible, as they included a sufficiently robust empirical evaluation of a tertiary prevention initiative using primary data. Narrative synthesis of the reviewed studies suggested that deradicalization interventions were harder to implement and less effective on average than disengagement/social reintegration interventions. This was echoed in the intervention modules that were most often described as successful: education, vocational training, and socialization components were preferred to religious education modules or online interventions purposed to challenge violent radical ideologies. The delivery of programs was facilitated by following the risk, needs, and responsivity principles of effective correctional intervention, as well as adequate training of practitioners, cooperation between the staff, good therapeutic alliance, complementary psychological counseling, and involving prosocial family members in the intervention. However, these conclusions rely on studies with substantial methodological limitations that hinder one’s confidence in their results. A screening of studies published between 2020 and 2024 was conducted and largely replicated the conclusions reported herein.