Maintaining the physical performance of cancer patients is increasingly considered due to the growing number of cancer patients and the aggressiveness of the treatments. For this reason, bioimpedance is now being used to record patients’ body composition by obtaining the phase angle (PhA). Although there is a direct relationship between PhA, age, sex and disease prognosis, it has not been measured as an analysis of physical performance in oncology patients and is a valid tool in the follow-up of cancer patients. For this purpose, 311 patients were evaluated, and both bioelectrical impedance analysis (BIA) and physical performance measurements were performed. The modification of the results concerning PhA was found to be highly relational, as a variation in one of the variables affected the other. It was concluded that each degree increase in PhA modified −22.57 s [−27.58; −17.53] in 400-m walking test (400 mWT); 13.25 kg [10.13; 16.35] in upper-body strength (UBS); 6.3 [4.95; 7.65] in lower-body strength (LBS); 1.55 mL/kg/min [0.98; 2.12] in VO_2peak; 6.53 Watts [3.83; 9.20] in ventilatory threshold 1 (VT1); 10.36 Watts [7.02; 13.64] in ventilatory threshold 2 (VT2). It was also noted that age was a factor that affected the relationship between PhA and 400 mWT; the older the age, the higher the relationship. PhA data has been shown to be highly correlated with physical performance. This is of great importance in clinical practice because a cancer patient’s physical performance levels can be assessed during treatment.