Background: Accumulating evidence has proposed phase angle (PhA) as a marker for assessing cellular integrity and nutritional status in ulcerative colitis (UC) patients; the aim of the study was to evaluate the efficacy of PhA in assessing nutritional status in patients with UC, investigating its potential as a biomarker of disease activity. Methods: We conducted a cross-sectional study in patients with UC and healthy controls. We determined PhA by electrical bioimpedance and categorized participants through bioelectrical impedance analysis. They were classified as normal PhA > 6.1° and low PhA < 6.1° in men and normal PhA > 5.6° and low PhA < 5.6° in women. Results: PhA was significantly lower in UC patients than in controls (5.8 ± 0.8 vs. 6.6 ± 0.7°; p < 0.001). Among UC patients, participants with low PhA showed a decrease in lean, dry mass (LDM) (p < 0.001), total body water (p = 0.008), and intracellular water (p = 0.005), accompanied by an increase in extracellular water (p = 0.001) compared to UC patients with normal PhA. Conclusions: PhA significantly decreases in UC patients compared to healthy controls and is even more reduced when UC is active. A cut-off point of <6.1 for men and <5.6 for women could be suitable for nutritional diagnosis in patients with UC, but it still needs to be validated.