Adapting Youth Nominated Support Team (Yst) To Prevent The Escalation Of Suicide Risk Among Youth On Probation - PROJECT SUMMARY Rates of suicide ideation and behaviors (SIB) are higher among youth involved in the juvenile justice system, compared to those in the general population. Most youth in the justice system are on probation (i.e., under community supervision) and live in the community, meaning their BH needs are identified in the probation system but must be addressed in the resource-limited community BH system. Concerningly, 50-80% of youth on probation with service need do not receive care, given barriers at multiple levels including the youth/their family (e.g., stigma, low perceived treatment need), and juvenile probation/BH systems (e.g., coordination problems; service shortages; long waitlists). Thus, there is an urgent need for interventions that address youths’ SIB and promote treatment uptake. The Youth Nominated Support Team (YST) program is a psychoeducational, social support intervention originally created as an adjunctive to standard BH treatment for youth with suicide risk following psychiatric hospitalization. In YST, youth nominate supportive adults from their social networks. The adults participate in a psychoeducation session and then provide support to the youth, while they (the adults) participate in weekly check-ins with a BH professional for three months. In a randomized controlled trial, participation in YST was associated with lower rates of all-cause mortality and reduced risk of death due to intentional self-injury, including suicide and drug overdoses of unknown intent. Natural mentorship interventions, in which youth identify mentors from their social networks, are associated with positive outcomes for probation youth (e.g., reduced recidivism). However, to our knowledge, there are no such social support programs targeting youth SIB and treatment uptake, tailored to the unique contexts and needs of probation youth. This may be partially due to limited understanding of barriers/facilitators to the implementation of such programs, which must address inner/outer context (e.g., youth/family, supportive adult, staff, organizational) and bridging factors (interagency collaboration) across probation and BH. Our project is therefore designed to address this gap. Consistent with the goals of PAR-21-131, we propose a hybrid (Type I) effectiveness-implementation study to adapt YST as an early intervention program (as opposed to post-hospitalization) to prevent escalation of SIB and increase probation youths’ treatment uptake, bridging them to care. Our study comprises three phases: 1) formative work with key stakeholders, 2) adaptation of YST through participatory methods to create YST- Probation (YST-P), and 3) a single-arm pilot to examine reductions in SIB (within-subject comparison), and increased treatment uptake (comparing YST-P participants to a propensity-matched, historical control). We will additionally explore theorized mechanisms of intervention action as well as implementation outcomes and barriers/facilitators to YST-P. Results from this work will inform a larger, fully powered effectiveness trial, as well as future studies leveraging youths’ existing social support networks to prevent SIB and bridge them to care.