Background: Posttraumatic stress disorder (PTSD) is associated with morbidity and mortality in affected populations. Cancer survivors experience PTSD at a rate higher than the general population, with young age and female gender identified as risk factors. While young women with breast cancer experience greater psychosocial distress in general following diagnosis (dx), little is known about PTSD in this population. Methods: Women dx’d with Stage I-III breast cancer at age ≤ 40 were surveyed as a part of a multi-site cohort study. Demographic, treatment, psychosocial characteristics (including self-reported psychiatric conditions and use of psychiatric medications prior to dx), anxiety and depression (Hospital Anxiety and Depression Scale), fear of recurrence (Lasry Scale), and social support (Medical Outcomes Study) were assessed within 1 year after dx. PTSD was measured at 30 months post-dx using the PTSD Checklist – Civilian Version; a score ≥ 50 was considered positive for PTSD. Univariable and stepwise multivariable logistic regression were used to evaluate factors associated with PTSD. Results: 572 women were eligible for this analysis; median age at dx was 37 (range: 17-40); 87% had Stage I or II cancer. 37/572 women (6.5%) met criteria for PTSD at 30 months. Lower educational attainment, less financial comfort, less social support, stage 2 (vs. 1) disease, receipt of chemotherapy, fear of recurrence, anxiety and depression, and psychiatric comorbidities were associated (p ≤ 0.05) with PTSD in univariable analyses. Chemotherapy (OR = 3.48, 95% CI = 1.09-11.06), anxiety by HADS (OR = 20.29, 95% CI = 7.83-52.53), and psychiatric comorbidities (OR = 4.22, 95% CI = 1.40-12.74) were associated with increased likelihood of PTSD, whereas college education (OR = 0.25, 95% CI = 0.10-0.59) and greater social support (OR = 0.41, 95% CI = 0.17-0.99) appeared to be protective in multivariable analyses. Conclusions: PTSD affects a minority of young breast cancer survivors, with the prevalence in our cohort similar to that seen in other breast cancer populations. Early identification of those at risk for developing PTSD is essential for the adequate treatment of affected women and for the improvement of health outcomes and quality of life in cancer survivors.