There have been concerns about the psychological impact of COVID-19-related stressors on young adults. However, there remains limited information regarding how psychosocial factors and coping strategies correlate with suicidal ideation in young adults during the pandemic. We studied a cross-sectional, observational online survey using a probability-based, nationally representative sample of U.S. young adults aged 18 to 29 (N = 1077). We performed weighted logistic regression to evaluate how self-isolation, social support and coping strategies (exposures) were associated with depression and suicidal ideation (outcomes), adjusting for age, gender, race, educational level and sexual orientation. The method of multiple imputations for addressing missing data was executed through chained equations. A total of 296 participants had depression, while 323 had suicidal ideation. Individuals who consistently self-isolated exhibited nearly threefold higher odds of depression compared to those without self-isolation. Social support was consistently protective against depression and suicidal ideation. Coping through positive reframing was protective against depression and suicidal ideation, whereas substance use, self-blame and behavioural disengagement had the opposite impact. Providing social support may help prevent suicidal ideation among young adults during the pandemic. Interventions that focus on developing young adults' coping strategies, such as through positive reframing, are recommended to develop positive and healthy relationships. Healthcare providers should advise against substance use, self-blame and behavioural disengagement.