Importance: Children who are placed in out-of-home care may have poorer outcomes in adulthood, on average, compared with their peers, but the direction and magnitude of these associations need clarification. Objective: To estimate associations between being placed in out-of-home care in childhood and adolescence and subsequent risks of experiencing a wide range of social and health outcomes in adulthood following comprehensive adjustments for preplacement factors. Design, Setting, and Participants: This cohort and cosibling study of all children born in Finland between 1986 and 2000 (N = 855 622) monitored each person from their 15th birthday either until the end of the study period (December 2018) or until they migrated, died, or experienced the outcome of interest. Cox and Poisson regression models were used to estimate associations with adjustment for measured confounders (from linked population registers) and unmeasured familial confounders (using sibling comparisons). Data were analyzed from October 2020 to August 2021. Exposures: Placement in out-of-home care up to age 15 years. Main Outcomes and Measures: Through national population, patient, prescription drug, cause of death, and crime registers, 16 specific outcomes were identified across the following categories: psychiatric disorders; low socioeconomic status; injuries and experiencing violence; and antisocial behaviors, suicidality, and premature mortality. Results: A total of 30 127 individuals (3.4%) were identified who had been placed in out-of-home care for a median (interquartile range) period of 1.3 (0.2-5.1) years and 2 (1-3) placement episodes before age 15 years. Compared with their siblings, individuals who had been placed in out-of-home care were 1.4 to 5 times more likely to experience adverse outcomes in adulthood (adjusted hazard ratio [aHR] for those with a fall-related injury, 1.40; 95% CI, 1.25-1.57 and aHR for those with an unintentional poisoning injury, 4.79; 95% CI, 3.56-6.43, respectively). The highest relative risks were observed for those with violent crime arrests (aHR, 4.16; 95% CI, 3.74-4.62; cumulative incidence, 24.6% in individuals who had been placed in out-of-home care vs 5.1% in those who had not), substance misuse (aHR, 4.75; 95% CI, 4.25-5.30; cumulative incidence, 23.2% vs 4.6%), and unintentional poisoning injury (aHR 4.79; 95% CI, 3.56-6.43; cumulative incidence, 3.1% vs 0.6%). Additional adjustments for perinatal factors, childhood behavioral problems, and traumatic injuries, including experiencing violence, did not materially change the findings. Conclusions and Relevance: Out-of-home care placement was associated with a wide range of adverse outcomes in adulthood, which persisted following adjustments for measured preplacement factors and unmeasured familial factors.