Although it has been proposed that the dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis may act as a possible pathway linking early life stress to psychosis, this relationship has not yet been fully confirmed. This study aimed to investigate the relationship between childhood adversity (CA), cortisol levels, and psychosis. Eighty-five patients with psychosis and 170 control subjects were enrolled in the study. CA was evaluated using the Florence Psychiatric Interview, and Childhood Experience of Care and Abuse Questionnaire. Positive symptoms (PS) were assessed using the Positive and Negative Syndrome Scale. Cortisol levels were evaluated in saliva samples. Patients experienced more CA and showed higher cortisol levels than controls. Patients with CA showed higher morning cortisol levels and more severe PS than those without CA. Patients with higher morning cortisol levels showed severe delusions. These findings suggest that both CA and dysregulation of the HPA axis could be related to psychosis.