© 2019 Elsevier Masson SAS Objectives: Previous research suggested isolated associations of physical and psychological factors with fibromyalgia severity. Integration of physical and psychological, experienced and observed, modifiable factors associated with fibromyalgia severity in a single model will reveal therapeutic paths toward less severity of disease. We aimed to examine an encompassing model of determinants of fibromyalgia severity. Methods: This observational, population-based cross-sectional study included 569 people with fibromyalgia. An integrative model of fibromyalgia severity was tested by using structural equation modelling. This model included 8 factors: resilience, catastrophizing, active lifestyle, declarative memory, subjective fitness, objective fitness, psychological distress, and physical fatigue. Results: Two core paths were associated with reduced fibromyalgia severity: 1) a psychological path connecting high resilience and low catastrophizing with low distress and 2) a physical path, connecting a more active lifestyle (directly and via high objective and subjective physical fitness) with low fatigue. Additional interconnecting paths especially suggested a connection from the psychological to physical path. Our model explained 83% of the fibromyalgia severity. Conclusions: The present model integrated the complexity of mutually influencing factors of fibromyalgia severity, which may help to better understand the disease. It emphasised the importance of: 1) physical factors and psychological factors and their interconnections, 2) patients’ experiences and clinical measurements, and 3) positive and negative signs such as physical fitness and distress. Future longitudinal and experimental research should aim at testing the causal direction of the associations in the model as well as the clinical implications suggested by the model. For instance, to reduce fatigue, exercise should enhance not only objective fitness but also fitness-related perceptions. Reducing distress and fatigue seems crucial for lowering fibromyalgia severity.