During a psychotic episode, patients frequently suffer from severe maladaptive beliefs known as delusions. Despite the abundant literature investigating the simple presence or absence of these beliefs, there exists little detailed knowledge regarding their actual content and severity at the onset of illness.This study reports on delusions during the initiation of indicated treatment for first-episode psychosis (FEP).Data were systematically collected from a sample of 636 patients entering a catchment-based early intervention service for FEP. The average severity and frequency of each delusional theme at baseline was reported with the Scale for the Assessment of Positive Symptoms. Delusional severity (globally and per theme) was examined across a number of sociodemographic and clinical variables.Delusions were present in the vast majority of individuals experiencing onset of FEP (94%), with persecutory (77.7%) being the most common theme. Persecutory delusions remained consistent in severity across diagnoses, but were more severe with older age at onset of FEP. No meaningful differences in delusional severity were observed across gender, affective versus non-affective psychosis, or presence/absence of substance use disorder. Globally, delusion severity was associated with anxiety, but not depression. Delusions commonly referred to as passivity experiences were related to hallucinatory experiences.This community sample offers a rare clinical lens into the severity and content of delusions in FEP. Although delusional severity was consistent across certain sociodemographic and clinical variables, this was not always the case. Future research should now consider the course of delusion themes over time.