Background: The frequency of bipolar spectrum in patients with conversion disorder is not well known, while it is important to be aware of comorbid bipolarity in the management of conversion disorder. Objectives: The study aimed to identify the frequency of bipolar spectrum disorder among people with conversion disorder and the factors associated with this comorbidity. Methods: The participants were 96 consecutive patients aged 18 - 65 years, referring to the emergency department or admitted to the psychiatric wards of two university general hospitals in Tehran, Iran, in 2019. Patients fulfilled the DSM-IV criteria for conversion disorder based on the structured clinical interview (SCID). The Mood Disorder questionnaire (MDQ) was used to identify bipolar spectrum disorder. Demographic characteristics and other relevant correlated factors were also recorded using a checklist. Data were analyzed using Pearson chi-square/Fisher’s exact test and logistic regression model. Results: The frequency of bipolar spectrum disorder based on the MDQ was 35.4% in patients with conversion disorder. The results of the logistic regression model using the Backward Conditional method showed that only two variables, namely antidepressant-induced mania/hypomania (OR = 39; CI 95%: 4.56 - 333.7) and history of hospitalization in the psychiatric ward (OR = 7.1, CI 95%: 2.05 - 24.65), remained as associated factors after controlling for other variables. Conclusions: Our study showed a higher frequency of bipolar spectrum disorder in patients with conversion disorder than did previous epidemiological studies in Iran and other countries. Such significant comorbidity should discourage potentially harmful off-label use of antidepressants for the treatment of conversion disorder instead of looking for other underlying psychiatric or social problems. Further research is suggested in this area.