Patients with Guillain–Barré syndrome (GBS) are at high risk for inadequate nutrition throughout their illness, yet the wider impact of malnutrition in this population remains unknown. Thus, the purpose of our study was to investigate the associations between nutritional status and functional status at admission to inpatient rehabilitation for GBS and to determine whether the admission phase angle, a biological marker of cellular health, is a prognostic indicator of functional improvement at the end of rehabilitation. The study included 27 participants recovering from GBS who screened positive for nutritional risk upon admission to rehabilitation. According to the Global Leadership Initiative on Malnutrition criteria, the majority of participants were classified as malnourished. A decreased phase angle was found in 93% (mean 3.7°, SD 1.3°). Lower phase angle was moderately associated with lower motor Functional Independence Measure (mFIM) at admission (r = 0.53, P = 0.005), suggesting that phase angle may be an indicator of functional status. By the end of rehabilitation, all participants improved functional independence and muscle strength, and the majority improved walking abilities. However, the correlation between admission phase angle and mFIM efficiency was not statistically significant (P = 0.3867). We conclude that malnutrition is significantly associated with low functional independence and muscle strength at admission. The inability of admission phase angle to predict functional improvement is probably due to the complex interactions between recovery from GBS and interventions provided during a comprehensive multidisciplinary rehabilitation for GBS, and also relatively small sample size.