ABSTRACT Youth suicide is an urgent public health concern. Emergency Departments (EDs) often serve as the first line of contact for large numbers of suicidal adolescents, many of whom do not link to outpatient treatment and continue to be at high risk for post-discharge suicidal crises (suicide attempts, ED visits due to suicidal ideation and/or behavior). Thus, there is a critical need for efficacious continuity of care strategies for adolescents at elevated suicide risk who transition from ED care. One such promising, yet understudied, approach is to intervene with parents of suicidal adolescents. Parents of these high-risk youth are at the forefront of suicide prevention and are tasked with implementing post-discharge suicide prevention recommendations (e.g., lethal means restriction, attending to suicide warning signs, providing support, encouraging healthy coping, etc.). At the same time, in addition to feeling overwhelmed, parents report low confidence in their ability to engage in recommended suicide prevention activities. This proposal seeks to develop and pilot an adaptive, text-based intervention for parents of suicidal youth transitioning from ED care. The intervention will be comprised of two texting components targeting interrelated domains: (1) parental provision of adolescent-focused (A-F) support to promote safety and well-being of suicidal adolescents and (2) parent-focused support (P-F) directed at enhancing parents’ own well-being. Because suicidal youth are highly heterogeneous—and parents’ caregiving challenges and stress may vary in the post-discharge period—a texting intervention should ideally be tailored to meet parents’ unique and changing needs. In particular, while all elements of A-F support are considered essential, P-F support may require greater personalization to optimize post-discharge outcomes. Designed to address the dynamically changing needs of individuals, Just-In-Time Adaptive Interventions (JITAIs) adapt the provision of mobile-based interventions to maximize outcomes while minimizing burden. To this end, the P-F texting component will include an embedded micro-randomized trial (MRT)—an experimental design used for developing high-quality JITAIs. Our primary aims are to: (1) finalize A-F and P-F texting components with parents’ input (N=25); (2) pilot the text-based intervention with 90 parents randomized to a control group or to one of two six-week intervention groups receiving either the A-F or the A-F plus P-F texting components; and (3) pilot the embedded MRT, wherein parents will be randomized twice daily to P-F message or no message condition, to inform the development of a JITAI. In addition to feasibility and acceptability, the proposed mechanism (parental self-efficacy) and relevant distal outcomes will be assessed over 2,- 6-, and 12-weeks post discharge. Setting the stage for a full-scale trial, this study offers innovation as the first to incorporate text messaging and an MRT to guide the devel...