Objectives: This study aimed to evaluate the fatigue in patients with undifferentiated spondyloarthritis [USpA], and its relationship with disease-specific variables, spinal mobility measures and healthy related quality of life [HRQOL]. Methods: Eighty patients with USpA and forty healthy subjects were included in this study. The multidimensional assessment of fatigue [MAF] and the generic instrument Short Form 36 [SF 36] were used in patient and control groups to assess fatigue and [HRQOL]. Fatigue was also assessed with the Bath ankylosing spondylitis disease activity index [BASDAI] fatigue item. The evaluation included the activity of the disease [BASDAI], functional status [Bath ankylosing spondylitis functional index], and visual analog scale [VAS] of axial and joint pain. Demographics and disease-related data were obtained. Results: Patients with USpA had higher scores in MAF total and in all MAF subgroup scales than controls. All SF-36 subgroups scores were also found to be significantly lower in patients. Severe fatigue experienced 60% in patients. MAF total scores were found to be significantly correlated with morning stiffness, BASDAI, Bath ankylosing spondylitis functional index (BASFI), BASDAI fatigue, VAS-axial, and SF-36 subgroups scores in patients [p < 0.05]. Conclusions: The patients with USpA defined significantly more fatigue and lower HRQOL when compared with healthy persons. MAF was found to be related with the clinical and functional status and health-related quality of life of patients with USpA.