Background High rates of posttraumatic stress disorder (PTSD) have been documented in war-affected populations. The prevalence of Complex PTSD (CPTSD) has never been assessed in an active war zone. Here, we provide initial data on war-related experiences, and prevalence rates of ICD-11 PTSD and CPTSD in a large sample of adults in Ukraine during the Russian war. We also examined how war-related stressors, PTSD, and CPTSD were associated with age, sex, and living location in Ukraine. Method Self-report data were gathered from a nationwide sample of 2004 adult parents of children under 18 from the general population of Ukraine approximately 6 months after Russia's invasion. Results All participants were exposed to at least one war-related stressor, and the mean number of exposures was 9.07 (range = 1–26). Additionally, 25.9% (95% CI = 23.9%, 27.8%) met diagnostic requirements for PTSD and 14.6% (95% CI = 12.9%, 16.0%) met requirements for CPTSD. There was evidence of a strong dose–response relationship between war-related stressors and meeting criteria for PTSD and CPTSD. Participants who had the highest exposure to war-related stressors were significantly more likely to meet the requirements for PTSD (OR = 4.20; 95% CI = 2.96–5.95) and CPTSD (OR = 8.12; 95% CI = 5.11–12.91) compared to the least exposed. Conclusions Humanitarian responses to the mental health needs of the Ukrainian population will need to take account of posttraumatic stress reactions. Education in diagnosing and treating PTSD/CPTSD, especially in the situation of a significant lack of human resources and continuing displacement of the population, is necessary.