Background: People undergoing quarantine have been identified as particularly vulnerable to poor mental health during the coronavirus disease 2019 (COVID-19) pandemic. Aims: To explore how the COVID-19 pandemic affected the mental health of people during the COVID-19 outbreak after the Iranian government’s social distancing plan. Methods: Data (N=1524) were collected using a cross-sectional web-based survey via social media. Demographic variables and psychological status were evaluated using the General Health Questionnaire, Stress Response Inventory, and Brunel Mood Scale. Individuals’ reported that their mental health had decreased during the pandemic. Four factors were associated with increased stress: (i) fear of getting sick, (ii) unclear quarantine duration, (iii) impaired daily activities, and (iv) reduced social communication. Results: One-third of participants reported having physical symptoms (32.7%), 47% reported anxiety, 72% reported social dysfunction, and 28.3% reported depression. Overall, approximately half of participants (52.2%) reported having symptoms of mental health disorders. Detrimental mental health characteristics were higher among those who were female, younger, and single. Conclusions: Health policies should be implemented to help reduce the psychological burden both during and after the Iranian government’s social distancing plan, especially among those who are female, single, younger, and those who have a relative with COVID-19.