Purpose: People with fibromyalgia experience a disagreement between patient-reported (i.e., subjective) and performance-based (i.e., objective) status. This study aimed to (i) corroborate the discordance between subjectively and objectively measured physical function and (ii) examine whether catastrophizing (worrying, pain magnifying, and helpless cognitions) and self-efficacy (believing capable to manage pain) are independently associated with this discordance. Methods: This population-based cross-sectional study included 405 women with fibromyalgia and 193 age-matched female controls. Participants completed the Pain Catastrophizing Scale, Chronic Pain Self-efficacy Scale, and physical functioning subscales of the Revised Fibromyalgia Impact Questionnaire and Short Form-36 (SF-36) health survey. Objective physical function was measured with the Senior Fitness Test battery. Subjective and objective physical functions were expressed as deviation from the general population in standard deviation (SD) units using means and SD of the control group. Results: In fibromyalgia, subjective physical function was worse than objective physical function (p < 0.001). Higher catastrophizing was consistently associated with greater discordance between subjective and objective physical function, while self-efficacy was only significantly associated with this discordance when subjective physical function was assessed by means of the SF-36. Conclusions: Subjective physical function is more impaired than objective physical function in fibromyalgia, yet both are markedly impaired. Catastrophizing cognitions are associated with this discordance. In particular, high catastrophizing may promote a feeling of reduced ability to do meaningful activities of daily living (i.e., restrictions) that people with fibromyalgia are actually able to. Therefore, catastrophizing should be assessed and potentially targeted when focusing on improving physical function in fibromyalgia.Implications for rehabilitation Rehabilitation should focus on physical exercise programs to help women with fibromyalgia to improve their reduced physical function. In rehabilitation settings, physical function of people with fibromyalgia should be evaluated by both subjective and objective assessments to fully understand physical functioning and to test the existence of discordance between both assessments. In case of a large discordance between subjective and objective physical function, a physical exercise program might be better complemented with cognitive management techniques to reduce catastrophizing and subjective physical dysfunction. When people with fibromyalgia experience high levels of catastrophizing, subjective assessments seem to be poor indicators of physical function.