Self-efficacy is assumed to account for significant variance in the treatment outcome of chronic pain patients. The aim of this study was to provide a German version of an approved measure of disease-related self-efficacy in fibromyalgia (FM) patients which assesses treatment outcomes and specific differences compared to other pain patients. The 8-item short-form of the arthritis self-efficacy scale was translated into German (ASES-D) and administered to 148 FM patients and 53 patients with rheumatoid arthritis (RA). In addition, similar cognitive constructs (locus of control, optimism/pessimism, and general self-efficacy) and disease-related variables (pain, functioning, depression, and coping) were assessed. The instrument was further applied to 43 FM patients who underwent interdisciplinary group therapy. Validation methods consisted of correlation, principal component analysis and difference testing between the disease groups. The instrument met good psychometric properties. Evidence for construct validity was provided. Self-efficacy was sensitive to changes and could be used in predicting the treatment outcome in FM patients. The German short-form ASES-D is a further step toward an internationally comparable assessment of disease-related self-efficacy in FM.