Objectives: Reliable and valid instruments are essential for understanding fatigue in occupational settings. This study analyzed the psychometric properties of the Portuguese version of the Swedish Occupational Fatigue Inventory (SOFI). Material and Methods: A cross-sectional study was conducted with 218 workers from an automotive industry involved in assembly tasks for fabrication of mechanical cables. Convergent and discriminant validity, internal consistency reliability and confirmatory factor analysis were performed. Results: Results showed adequate fit to data, yielding a 20-item, 5-factor structure (all intercorrelated): Chi²/df (ratio Chi² and degrees of freedom) = 2.530, confirmatory fit index (CFI) = 0.919, goodness of fit index (GFI) = 0.845, root mean square error of approximation (RMSEA) = 0.084. The SOFI presented an adequate internal consistency, with the sub-scales and total scale presenting good reliability values (Cronbach’s α values from 0.742 to 0.903 and 0.943 respectively). Conclusions: Findings suggest that the Portuguese version of the SOFI may be a useful tool to assess fatigue and prevent work-related injuries. In future research, other instruments should be used as an external criterion to correlate with the SOFI dimensions. Int J Occup Med Environ Health 2017;30(3):407–417