Importance Children placed in out-of-home care (OHC) have higher rates of suicidal behaviors, including suicide attempts, compared with those who grow up in their family of origin. Several studies have shown that this elevated risk persists into young adulthood. Yet, our knowledge about any longer-term associations of OHC with suicide attempts is limited. Objective To examine how childhood experiences of placement in OHC are associated with trajectories of hospitalization because of suicide attempts (HSA) from early into late adulthood. Design, Setting, and Participants This prospective birth cohort study that was conducted in Stockholm, Sweden, and analyzed in March 2020 included 14559 individuals born in 1953 who were living in the greater metropolitan of Stockholm in November 1963 and followed through registers up until December 2016. Exposures Childhood experiences of OHC based on information from the Social Register (age 0-19 years). Main Outcomes and Measures Hospitalization because of suicide attempts based on in-patient care data from the National Patient Register. Group-based trajectory modeling was used to cluster individuals according to their probabilities of HSA across adulthood (age 20-63 years). Results In this cohort of 14559 individuals (7146 women [49.1%]), 1320 individuals (9.1%) had childhood experiences of OHC, whereas 525 individuals ( 3.6%) had HSA. A Cox regression analysis showed a substantially higher risk of HSA among those with childhood experiences of OHC (hazard ratio, 3.58; 95% CI, 2.93-4.36) and after adjusting for a range of adverse childhood living conditions (hazard ratio, 2.51; 95% CI, 2.00-3.15). Those with at least 1 HSA were grouped into 4 trajectories: (1) peak in middle adulthood (66 [12.6%]), (2) stable low across adulthood (167 [31.8%]), (3) peak in early adulthood (210 [40.0%]), and (4) peak in emerging adulthood (82 [15.6%]). A multinomial regression analysis suggested that those with experiences of OHC had higher risks of following any of these trajectories (trajectory 1: relative risk ratio [RRR], 2.91; 95% CI, 1.61-5.26; trajectory 2: RRR, 3.18; 95% CI, 2.21-4.59; trajectory 3: RRR, 4.32; 95% CI, 3.18-5.86; trajectory 4: RRR, 3.26; 95% CI, 1.94-5.46). The estimates were reduced after adjusting for adverse childhood living conditions. Conclusions and Relevance The findings suggest that the elevated risk of suicide attempts among former child welfare clients does not cease after young adulthood, indicating the necessity for clinical attention to childhood experiences of OHC as a risk marker for suicidal behavior across the life span. Question How are childhood experiences of placement in out-of-home care associated with trajectories of hospitalization because of suicide attempts from early into late adulthood? Findings In this cohort study of 14559 individuals, individuals were grouped into 4 trajectories with differential onset of suicide attempts across adulthood. Childhood experiences of placement in out-of-home care were associated with increased risks of following each of these trajectories. Meaning The elevated risk of suicide attempts among former child welfare clients persists into young adulthood, indicating the necessity for clinical attention to childhood experiences of out-of-home care as a risk marker for suicidal behavior across the life span. This cohort study examines the association between childhood experiences of placement in out-of-home care and trajectories of hospitalization because of suicide attempts from early into late adulthood in Sweden.