Background People with mild to borderline intellectual disabilities (MBIDs) face challenges in social functioning, possibly as a result of limited mentalising abilities such as reflecting on the behaviour of themselves and others. Reflective functioning in people with MBIDs has not yet been investigated due to a lack of instruments. The Reflective Functioning Questionnaire (RFQ) is a seemingly easy adaptable, short self-report questionnaire. The aim of the present, explorative study was to adapt the RFQ for people with MBIDs and investigate the psychometric properties and correlations with other mentalising related constructs. The formulation of the items was adapted to the target group and items were added to broaden the scope towards reflection on both the self and other. Method Participants were 159 adults with MBIDs who completed a Dutch-translated and easy-to-read RFQ with five supplemental items, a questionnaire for autistic traits, a self-report questionnaire assessing perspective taking and two performance-based measures assessing emotion recognition and Theory of Mind. Results Confirmatory factor analysis confirmed the factor structure of the RFQ and revealed a two-factor structure with a Self and Other subscale. Generally satisfactory internal consistency and test-retest reliability were found. Explorative results showed correlations of the RFQ-8 and RFQ subscales with autistic traits and between the RFQ Other and perspective taking. Conclusions This explorative study is the first testing psychometric properties of the RFQ as a self-report questionnaire for assessing reflective functioning in adults with MBIDs. This step is relevant in gaining more scientific knowledge on assessing mentalising in people with MBIDs.