Posttraumatic stress disorder (PTSD) has been associated with impaired educational performance. Previous studies on the disorder could not control for important measured and unmeasured confounders. To prospectively investigate the association between PTSD and objective indicators of educational attainment across the life span, controlling for familial factors shared by full siblings, psychiatric comorbidity, and general cognitive ability. This population-based cohort study included 2 244 193 individuals born in Sweden between January 1, 1973, and December 31, 1997, who were followed-up until December 31, 2013. Clusters of full siblings were used to account for familial factors. Data analyses were conducted between December 2018 and May 2020. International Classification of Diseases, Ninth Revision and International Statistical Classification of Diseases and Related Health Problems, Tenth Revision diagnoses of PTSD in the Swedish National Patient Register. Eligibility to access upper secondary education after finishing compulsory education, finishing upper secondary education, starting a university degree, and finishing a university degree. Of the final cohort of 2 244 193 individuals (1 151 414 [51.3%] men) included in the analysis, 1 425 326 were assessed for finishing compulsory education (919 with PTSD), 2 001 944 for finishing upper secondary education (2013 with PTSD), and 1 796 407 and 1 356 741 for starting and finishing a university degree (2243 and 2254 with PTSD, respectively). Posttraumatic stress disorder was associated with lower odds of achieving each of the educational milestones during the study period, including 82% lower odds of finishing compulsory education (adjusted odds ratio [aOR], 0.18; 95% CI, 0.15-0.20), 87% lower odds of finishing upper secondary education (aOR, 0.13; 95% CI, 0.12-0.14), 68% lower odds of starting a university degree (aOR, 0.32; 95% CI, 0.28-0.35), and 73% lower odds of finishing a university degree (aOR, 0.27; 95% CI, 0.23-0.31). Estimates in the sibling comparison were attenuated (aOR range, 0.22-0.53) but remained statistically significant. Overall, excluding psychiatric comorbidities and adjusting for the successful completion of the previous milestone and general cognitive ability did not statistically significantly alter the magnitude of the associations. Posttraumatic stress disorder was associated with educational impairment across the life span, and the associations were not entirely explained by shared familial factors, psychiatric comorbidity, or general cognitive ability. This finding highlights the importance of implementing early trauma-informed interventions in schools and universities to minimize the long-term socioeconomic consequences of academic failure in individuals with PTSD.