There were nearly 12 million admissions to US jails in 2012. Jailed individuals face a disproportionate risk for suicide. The time of arrest and jail detention represents an acute stressor that further exacerbates this risk. The epidemic of suicide during jail detention has been recognized. However, less attention has been paid to the high suicide risk and mortality in the months following jail release, as individuals re-enter their communities, are faced with financial, legal, and social stressors, and have increased access to lethal means (e.g., drugs, cars, firearms). Given that roughly 10% of all suicides in the U.S. with known circumstances occur following a recent criminal legal stressor (often arrest and jail detention), reducing suicide risk in the year after jail detention could have a noticeable impact on national suicide rates. Unlike prison, where individuals have already been sentenced and typically stay from months to years, most pretrial jail detainees are released within days. Therefore, brief interventions are required. Stanley and Brown's Safety Planning Intervention (SPI) is a brief, adjunctive suicide risk reduction intervention developed for suicidal patients presenting to urgent care settings. SPI incorporates evidence-based suicide risk reduction strategies in a low intensity, low cost intervention that can be delivered by a broad range of clinicians, making it scalable in the mental health resource-poor justice system. SPI reduces subsequent suicidal ideation and attempts among at-risk individuals in emergency rooms. However, there is no previous test of this intervention (or any other) for reducing suicidality following jail release. This RCT evaluates the effectiveness and cost-effectiveness of SPI for reducing suicide events (attempts, suicide behaviors, and suicide-related hospitalizations and emergency department visits) and attempts among 800 suicidal pretrial jail detainees from two jails in the year following jail release. It assesses critical mechanisms of suicide reduction in our target population: treatment utilization, suicide-related problem-solving, and belongingness. SPI will consist of safety planning during jail detention and follow-up phone sessions after jail release. Research with previously incarcerated individuals and with suicidal individuals in the community has shown that telephone follow-up intervention is feasible and powerful in building trust and reducing risk among these disenfranchised, isolated populations. This study will be the first randomized evaluation of a suicide prevention intervention in the vulnerable year after jail release. Beyond the human suffering and costs at an individual level, suicidal behavior incurs high economic costs in terms of health care costs and lost productivity. Jails are a catchment area for at-risk individuals at a time of high life stress and high suicide risk, providing an important opportunity for suicide prevention intervention, one that is currently being missed. This study will provide the data on costs and cost-adjusted outcomes that systems need to make informed decisions about adoption, speeding implementation. Thus, this study will contribute to knowledge about both mechanisms of action and system-level intervention effects.