Purpose To evaluate physical fitness and its association with fatigue in patients with low grade glioma (LGG). Methods Cross-sectional study. Muscle strength was measured with a digital dynamometer, cardiorespiratory fitness (peak oxygen uptake (VO2peak), maximal workload (MWL)) by cardiopulmonary-exercise-testing, and fatigue by using the Multidimensional Fatigue Inventory. Results Thirty patients were included, mean age of 44.1 (SD11.2) years, and 67% were men, 31.2 (SD18) months post-diagnosis. Muscle strength (p < 0.01), and cardiorespiratory fitness (VO2peak, MWL) (p < 0.01) were significantly decreased compared to predicted values based on age and gender. Thirty percent of the patients experienced severe physical fatigue, and severe mental fatigue was reported in 57% of the patients. Cardiorespiratory fitness showed weak to moderate (r − 0.46 to r − 0.52) but significant (p < 0.01) correlations with physical fatigue, not with mental and general fatigue. Muscle strength was not associated with fatigue. A lower VO2peak was independently associated with a higher level of physical fatigue, adjusted for Karnofsky Performance Status (R2 0.40). Conclusions Physical fitness (muscle strength, cardiorespiratory fitness) is reduced in patients with LLG, and a lower level of cardiorespiratory fitness (VO2peak) is independently associated with a higher level of experienced physical fatigue. Trials to explore the benefit of exercise programs to improve cardiorespiratory fitness and, consequently, fatigue are warranted. Implications for rehabilitation Physical fitness (muscle strength and cardiorespiratory fitness) is reduced in patients with low-grade glioma. Patients with low-grade glioma should be screened for fatigue with the multidimensional fatigue inventory, to differentiate between mental and physical fatigue. Patients with low-grade glioma with severe physical fatigue should be screened for reduced physical fitness, especially cardiorespiratory fitness by objective cardiopulmonary-exercise-testing. Rehabilitation exercise programs to improve cardiorespiratory fitness and, consequently, (physical) fatigue could be warranted in patients with low-grade glioma.