Introduction Religious delusions have regularly been reported throughout history in patients with schizophrenia, affective psychoses, complex partial seizure disorders and drug-induced psychoses. Objectives The purpose of the study was to identify potential differences regarding demographic and clinical features of bipolar and schizoaffective patients presenting religious delusions. Method Thirty-five inpatients diagnosed with bipolar disorder and 32 inpatients diagnosed with schizoaffective disorder, according to ICD 10 diagnostic criteria, were selected for this study. The assessment of the subjects took place between 2012 and 2015. We divided the subjects into two samples: with and without religious delusions. Socio-demographic (age, sex, education, marital status) and clinical data were analysed. The Brief Psychiatric Rating Scale (BPRS) and the Global Assessment of Functioning Scale (GAF) were used to assess current psychopathology and global functioning. Results Among the schizoaffective patients, 17.8% and 16.9% of bipolar patients presented religious delusions. Similarly, in both samples, patients with religious delusion were older, more frequently unmarried women, had graduated high school, had higher BPRS total scores and higher BPRS psychotic cluster scores and had lesser global functioning. Conclusions Religious delusions were not related to any of the two diagnostic groups but were associated, in both bipolar and schizoaffective patients, with an increased severity of psychotic positive symptoms and had worse global functioning.