Background: Guillain–Barré Syndrome (GBS) is one of the most severe neurological diseases that causes marked disability and even death. Aim: The aim of this study is to investigate the role of the neutrophil–lymphocyte ratio (NLR) as a prognostic marker for GBS and response to treatment with plasmapheresis. Methods: Seventy-five subjects (35 GBS patients and 40 healthy controls) were recruited. Complete general and neurological examinations were performed and Hughes disability scale score was evaluated for assessing functional motor deficits in GBS patients. In addition, NLR, erythrocyte sedimentation rate, and C-reactive protein level were calculated. Results: NLR was significantly higher in GBS patients than in controls (p < 0.001) and was significantly higher in axonal form than other demyelinating and mixed subtypes (p < 0.02). Patients with a poor outcome had a significantly high NLR than patients with a good outcome (p = 0.006). NLR was also positively correlated with Hughes disability scale score (p < 0.001). The cut-off value for NLR to predict a good response of patients to plasmapheresis was ≤ 4.4. Interpretation: NLR may be a rapid, simple, inexpensive biomarker for predicting the severity of GBS, outcome of patients, and their response to plasmapheresis.