Cancer patients suffer from metabolic and pathophysiological changes that contribute to malnutrition. These metabolic changes lead to loss of cell integrity, which induces dehydration intracellular and increase extracellular fluid. Bioelectrical impedance analysis (BIA)-derived phase angle (PhA) is considered a good tool to evaluate hydration status, but in cancer patients it is not fully elucidated. Thus, in cancer patients the aims of this study were to (1) verify the association between PhA and fatigue, (2) verify the association between PhA and fatigue after adjustment for extracellular fluid accumulation, and (3) assess the prevalence of fatigue. This cross-sectional study was conducted with 124 patients of both genders on cancer treatment. Body weight, height, body mass index, handgrip strength, performance status, and cachexia were collected. In addition, body composition was evaluated by BIA to obtain hydration status and PA. The cut-off point used to classify patients with low PhA was set <4°. To identify fatigue, the Functional Assessment of Cancer Therapy Fatigue questionnaire was applied. Of the 124 patients evaluated (n = 98/79% men), 26% had fatigue. The prevalence of fatigue was higher in patients with lower PhA <4° (65.63%). In the logistic regression analyses, we found that patients with PhA >4° had lower risk for fatigue (OR: 0.92 95% CI [0.86-0.99], p = 0.03) in the crude model, however after adjustments by weight loss percentage in 6 months, age, sex, and hydration the association was not maintained (OR: 0.94 95% CI [0.85-1.04], p = 0.26). In conclusion, we found that ~26% of cancer patients have fatigue. In spite of adjustment for extracellular fluid, PhA is not associated with fatigue. The importance of measuring PhA to assess intra and extracellular hydration in cancer patients is highlighted.