BACKGROUND: Carers play an important role in providing practical and emotional support to people with dementia. There is a need to ensure carers are supported in this role to maximise their ability to provide care while maintaining their own wellbeing. This study aimed to develop a psychometrically rigorous self-report instrument to assess the unmet needs of carers of people with dementia. METHODS: The Unmet Needs Instrument for Carers of People with Dementia (UNI-C) was developed using a multi-methods approach including a comprehensive literature review, in-depth interviews and focus groups, expert opinion, cognitive interviews and pilot testing. A cross-sectional survey of 169 carers was used to examine the psychometric characteristics of the instrument. Specifically, internal reliability, and structural and convergent validity were assessed and the number of items reduced. RESULTS: The original 80 item instrument was reduced to 46 items across four domains based on prevalence, item-total and pairwise correlations, factor loadings and clinical relevance. The resulting instrument, named the UNI-C46, showed acceptable evidence of structural and convergent validity, and good internal reliability (α = 0.94) and acceptability. The four domains measure 'Your own wellbeing' (16 items), 'Finding information' (12 items), 'Managing practical needs' (10 items), and 'Managing dementia symptoms' (8 items). CONCLUSIONS: This study provides the first demonstration of reliability and validity of the UNI-C46. Further testing of these properties with a larger and more representative sample of carers is recommended.