Morphing fears (also called transformation obsessions) involve concerns that a person may become contaminated by and acquire undesirable characteristics of others. These symptoms are found in patients with obsessive–compulsive disorder (OCD) and are thought to be related to mental contamination. Given the high levels of distress and interference morphing fears can cause, a reliable and valid assessment measure is needed. This article describes the development and evaluation of the Morphing Fear Questionnaire (MFQ), a 13‐item measure designed to assess for the presence and severity of morphing fears. A sample of 900 participants took part in the research. Of these, 140 reported having a current diagnosis of OCD (SR‐OCD) and 760 reported never having had OCD (N‐OCD; of whom 24 reported a diagnosis of an anxiety disorder and 23 reported a diagnosis of depression). Factor structure, reliability and construct and criterion‐related validity were investigated. Exploratory and confirmatory factor analyses supported a one‐factor structure replicable across the N‐OCD and SR‐OCD group. The MFQ was found to have high internal consistency and good temporal stability and showed significantly greater associations with convergent measures (assessing obsessive–compulsive symptoms, mental contamination, thought–action fusion and magical thinking) than with divergent measures (assessing depression and anxiety). Moreover, the MFQ successfully discriminated between the SR‐OCD sample and the N‐OCD group, anxiety disorder sample and depression sample. These findings suggest that the MFQ has sound psychometric properties and that it can be used to assess morphing fear. Clinical implications are discussed. Copyright © 2015 John Wiley & Sons, Ltd. Key Practitioner Message Little remains known about morphing fears, but it is an important area of investigation due to symptoms being highly distressing and often debilitating Because morphing fears commonly present as obscure symptoms, they may not be recognized as a type of OCD The MFQ is a robust measure with clinical utility; it can facilitate recognition and assessment of morphing fears The MFQ will allow for further investigations of the prevalence, correlates and treatment outcomes of morphing fears.