Current policy in UK health services emphasizes that, where possible, people with intellectual disabilities should access the same services as people without intellectual disabilities. One of the barriers to this is the confidence of clinicians and therapists. In this paper, we report on the development of a scale to describe the confidence of therapists in working with people with intellectual disabilities (the Therapy Confidence Scale–Intellectual Disabilities [TCS‐ID]). One‐hundred and eighty‐one therapists who provided talking therapies but who did not work primarily with people with intellectual disabilities completed the scale; 43 people completed the scale twice for test–retest reliability purposes. One‐hundred and seven people also completed a scale of general therapy self‐efficacy. The TCS‐ID has a single factor structure accounting for 62% of the variance, Cronbach's alpha for the scale is 0.93 and test–retest reliability is 0.83. There are significant differences in confidence based upon participants experience in working with people with intellectual disabilities and their therapeutic orientation, and there is a significant association between the TCS‐ID and the General Therapy Self‐efficacy Scale. Sixty clinicians working in mainstream mental health services received training on adapting their therapeutic approaches to meet the needs of people with intellectual disabilities. The TCS‐ID was used pre‐training and post‐training and demonstrated a significant increase in confidence for all group. We suggest that the scale has good psychometric properties and can be used to develop an understanding of the impact of training for mainstream therapist in working with people with intellectual disabilities. Copyright © 2014 John Wiley & Sons, Ltd. Key Practitioner Message There is increasing emphasis on people with intellectual disabilities receiving services from mainstream mental health services. There is no research describing the experiences and outcomes of people with intellectual disabilities receiving mainstream talking therapy services. The confidence that clinicians have in working with people with intellectual disabilities may be a barrier to them receiving mainstream services. The Therapy Confidence Scale–Intellectual Disabilities is a psychometrically sound scale for measuring the confidence of therapists in working with people with intellectual disabilities and is a useful outcome measure for training clinicians to work with people with intellectual disabilities.