This article summarizes the development and validation of a scale to measure the level of self-efficacy of patients with type 2 diabetes mellitus. Self-efficacy is described as people’s belief in their capability to organize and execute the course of action required to deal with prospective situations. This self-efficacy scale was developed based on the self-care activities these patients have to carry out in order to manage their diabetes. The following psychometric properties of this scale were established: content validity, construct validity, internal consistency and stability. The original scale contained 42 items. A panel of five experts in diabetes and four self-efficacy experts evaluated the original scale two times for relevance and clarity. This content validity procedure resulted in a final scale which consisted of 20 items. Subsequently, patients with type 2 diabetes were asked to complete this 20-item scale and further tests were done with the 94 usable responses. Factor analysis identified four factors, all of which were related to clusters of self-care activities used to manage diabetes which comprised this scale. The internal consistency of the total scale was α=0·81 and the test–retest reliability with a 5-week time interval was r=0·79 (P < 0·001).