The most frequently reported symptoms by patients with ankylosing spondylitis (AS) are pain, stiffness, and fatigue. Previous studies have estimated a 63% prevalence of fatigue in AS, with a low correlation of fatigue with pain and functional capacity. The objective of this study is to assess fatigue prevalence in AS patients and establish the main associated factors. A case-control study including AS patients according to New York modified criteria was carried out. The control group included individuals of the general population without rheumatic conditions, matched by gender, age, and socioeconomic level. Disease-related variables were recorded. Functional capacity, disease activity, and quality of life were assessed using Bath Ankylosing Spondylitis Funcional Index (BASFI), Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), and ankylosing spondylitis quality of life (ASQoL). CES-D questionnaire was used to evaluate depression and fatigue severity scale (FSS) to evaluate fatigue. Sixty-four consecutive AS patients and 95 controls were included. Patients' median age was 44 years (interquartile range (IQR), 33.25-53), 89.1% were male, and had a median disease duration of 17 years (IQR, 10.3-25). Fatigue prevalence in AS was 73.4% compared to 30.5% in the control group (p < 0.001; OR, 2.08 (95% CI, 1.53-2.83)). Furthermore, fatigue in AS correlated with ASQoL (r = 0.65), BASFI (r = 0.52), BASDAI (r = 0.52), and depression (r = 0.51), whereas no correlation with age or disease duration was found. In the linear regression analysis using fatigue as the dependent variable, depression was the only associated variable (p = 0.01). No association with age, gender, disease duration, BASDAI, BASFI, or presence of comorbidities was found. Finally, BASDAI fatigue question correlated with the FSS (r = 0.55). Fatigue was significantly more prevalent in AS than in healthy controls. The main determinant factor of fatigue was the presence of depression, explaining 30% of its variability. Recognizing these factors could have important therapeutic implications.