This study investigated whether individual differences in obsessive-compulsive symptoms and in thought–action fusion are related to theory of mind abilities. One hundred and ninety-two adult participants completed self-reports of obsessive–compulsive symptoms (OCI-R), thought–action fusion (TAF), private self-consciousness (PSC) and self-reflectiveness (SR) as mentalising abilities, and anxiety and depression. A nonintrospective method examining participants' implicit structure of their lexicon for ‘knowing’ was used to assess theory of mind. Private self-conciousness and SR added to the prediction of OCD symptoms independently of TAF and depression but did not mediate the relationship between TAF and OCD symptoms. Participants high in thought–action fusion gave a greater emphasis to the certainty dimension of the mental lexicon and placed lesser importance on the source of information dimension than those low in TAF. Our results provide preliminary evidence of a relationship between theory of mind and thought–action fusion. People disposed to thought–action fusion are more likely to make a significance judgment about ‘knowing’ based on the degree of certainty than on reference to the source of knowledge. Identifying disruptions to theory of mind abilities in OCD provides links to solid theory and evidence about metacognitive development and may help integrate cognitive processing and cognitive appraisal models of OCD.