Background: Loss of companion animals can result in various psychological reactions, including complicated grief, depression, anxiety, and insomnia. However, research on the prevalence of patients requiring clinical intervention is limited. Moreover, research examining the relationship between childhood trauma and psychological symptoms after pet loss is lacking. Therefore, this study aimed to investigate the rates of clinically significant psychological distress after pet loss and the impact of childhood trauma on adults who have experienced pet loss. Methods: An online survey was conducted with non-clinical adult participants who had experienced the loss of a companion animal. Psychiatric characteristics were evaluated using standardized assessments, including the Inventory of Complicated Grief (ICG), the Patient Health Questionnaire-9 (PHQ-9), the Generalized Anxiety Disorder-7 (GAD-7), and the Insomnia Severity Index (ISI). Childhood trauma was evaluated using the Childhood Trauma Questionnaire (CTQ). Results: In a sample of 137 individuals who had experienced pet loss, the percentages of those who exceeded the cutoff points were 55% for the ICG, 52% for the PHQ-9, 40% for the GAD-7, and 32% for the ISI. The group that reported childhood trauma exhibited significantly higher scores on the ICG (t = 2.16, P = 0.032), PHQ-9 (t = 3.05, P = 0.003), GAD-7 (t = 2.61, P = 0.010), and ISI (t = 2.11, P = 0.037) than in the group without childhood trauma.For participants who experienced pet loss for less than one year, there was no significant difference in the ICG between the trauma and non-trauma groups, as both had extremely high scores. However, the trauma group had a significantly higher PHQ-9 (t = 2.58,P = 0.012) than the non-trauma group. In contrast, for participants who experienced pet loss for more than one year, the trauma group had a significantly higher ICG (t = 2.22, P = 0.03) than the non-trauma group, while there was no significant difference in the PHQ-9. Additionally, the emotional abuse scores on the CTQ were most significantly correlated with the ICG scores even after controlling for depressive symptoms. Conclusion People who experienced pet loss had significant psychological symptoms, ranging from 32% to 55%. Childhood trauma experiences affect complicated grief, depressive symptoms, anxiety symptoms, insomnia and prolonged grief disorder after pet loss. After pet loss, people with childhood trauma may require more psychological help than those without trauma.