Importance: The psychological symptoms associated with having a family member admitted to the intensive care unit (ICU) during the COVID-19 pandemic are not well defined. Objective: To examine the prevalence of symptoms of stress-related disorders, primarily posttraumatic stress disorder (PTSD), in family members of patients admitted to the ICU with COVID-19 approximately 90 days after admission. Design, Setting, and Participants: This prospective, multisite, mixed-methods observational cohort study assessed 330 family members of patients admitted to the ICU (except in New York City, which had a random sample of 25% of all admitted patients per month) between February 1 and July 31, 2020, at 8 academic-affiliated and 4 community-based hospitals in 5 US states. Exposure: Having a family member in the ICU with COVID-19. Main Outcomes and Measures: Symptoms of PTSD at 3 months, as defined by a score of 10 or higher on the Impact of Events Scale 6 (IES-6). Results: A total of 330 participants (mean [SD] age, 51.2 [15.1] years; 228 [69.1%] women; 150 [52.8%] White; 92 [29.8%] Hispanic) were surveyed at the 3-month time point. Most individuals were the patients' child (129 [40.6%]) or spouse or partner (81 [25.5%]). The mean (SD) IES-6 score at 3 months was 11.9 (6.1), with 201 of 316 respondents (63.6%) having scores of 10 or higher, indicating significant symptoms of PTSD. Female participants had an adjusted mean IES-6 score of 2.6 points higher (95% CI, 1.4-3.8; P <.001) than male participants, whereas Hispanic participants scored a mean of 2.7 points higher compared with non-Hispanic participants (95% CI, 1.0-4.3; P =.002). Those with graduate school experience had an adjusted mean score of 3.3 points lower (95% CI, 1.5-5.1; P <.001) compared with those with up to a high school degree or equivalent. Qualitative analyses found no substantive differences in the emotional or communication-related experiences between those with high vs low PTSD scores, but those with higher scores exhibited more distrust of practitioners. Conclusions and Relevance: In this cohort study, symptoms of PTSD among family members of ICU patients with COVID-19 were high. Hispanic ethnicity and female gender were associated with higher symptoms. Those with higher scores reported more distrust of practitioners..