Early mobilisation is beneficial to support recovery among critically ill patients. The literature highlights the benefits of family engagement in early mobilisation, yet this practice remains underutilised. Effective implementation depends on understanding the key antecedents that influence family engagement in early mobilisation, specifically families' knowledge, contemplation, confidence and readiness. However, no measurement tools currently exist to assess these. Therefore, developing a psychometrically supported instrument is essential to understanding and enhancing families' factors influencing their engagement in early mobilisation. To develop and evaluate the psychometric properties of an instrument to assess families' knowledge, contemplation, confidence and readiness to engage in early mobilisation. A multi-site cross-sectional survey design. Based on established psychological theory (Social Cognitive Theories and Behaviour Change Theories), an item pool was developed to assess families' knowledge, contemplation, confidence and readiness to participate in early mobilisation. To psychometrically evaluate the new tool, a multi-site cross-sectional survey was undertaken from May 2020 to June 2022 across five intensive care units in Australia. Data from 370 families of critically ill patients were used to evaluate the structural, convergent and discriminant validity as well as the reliability of the new instrument. Confirmatory factor analysis indicated good model fit, supporting the proposed structure. All items displayed high standardised factor loadings except one, which improved upon freeing an error covariance. Positive inter-factor correlations were moderate to strong and were substantially lower than the square root of the average variance extracted, supporting both convergent and discriminant validity, respectively. Additionally, all subscales demonstrated well to excellent reliability. The findings provide preliminary support for the multiple types of validity evidence and the reliability of the instrument. This new instrument is suitable for use in clinical and research applications to assess families' knowledge, contemplation, confidence and readiness for their engagement in early mobilisation. Family engagement in early mobilisation activities may have multiple benefits but it is not commonly implemented in the ICU. Factors influencing family engagement in early mobilisation are poorly understood. Influential psychological theories highlight the likely importance of knowledge, contemplation, confidence and readiness. A readily available instrument designed to assess these constructs among family members is needed to deepen research understanding and guide clinical practice. The proposed instrument is designed to measure factors influencing family engagement in early mobilisation, which may support healthcare professionals and health services to identify and tailor strategies to support family engagement in early mobilisation. Recommendations for reporting the results of studies of instrument and scale development and testing was followed to report this study. Family members of adult critically ill patients participated in this study, and they provided the data through the survey.