The primary aim of this report is to discuss a case of delusional disorder with jealousy delusions of incestuous content where a father and an uncle sharing similar delusions. Our case is a 37-year-old male patient, whose complaints had started 4 years back when he moved out from his house upon thinking that two siblings living in his neighborhood were having an affair. After that he developed jealousy delusions that his wife was cheating on him with her brother. Since his delusions kept increasing in the past 6 months, his family and social relations began to break down. He started to put camera and voice recorders in the house in order to prove his ideas and he stopped going to work and even going out because of his jealousy delusions. So he was brought to our clinic and hospitalized. He had no history of psychiatric or organic disease. According to the history taken from the patient and his relatives, patient’s father and uncle had similar jealousy delusions about their wives at similar ages and were hospitalized many times. In the psychiatric examination, the patient was conscious, oriented, and cooperative and had good self-care and an age-appropriate look. His psychomotor activity was normal. He was partially open to communication. He had a dysphoric mood and an anxious affect. He did not describe any hallucinations. He had jealousy delusions regarding an incestuous affair between his wife and her brother. His functioning was poor because of his delusions. Cognitive functions were preserved. His impulse control was poor. His judgment was also poor and insight was partial. There was no homicidal or suicidal ideation in thought content. Results of routine biochemical tests, complete blood count and thyroid hormone profile were within a normal range. Cranial CT and EEG findings were normal. Rorschach test could not be evaluated because of inadequate data due to patient’s defensive attitude. Risperidone 4 mg/day and alprazolam 0.5 mg/day were started with a diagnosis of delusional disorder. Patient whose delusions were partially improved was discharged on his and his family’s request to carry on an outpatient treatment. Delusional disorder is a psychiatric condition that starts in middle ages but also one can display paranoid features in premorbid personality. As with our case, similar jealousy delusions in the family may suggest the role of genetic predisposition as well as premorbid personality traits.