This study evaluated the diagnostic value of magnetic resonance imaging (MRI) and computed tomography (CT) in prevertebral space invasion in patients with head and neck squamous cell carcinomas (HNSCC). This study retrospectively reviewed 197 patients with advanced primary laryngeal or hypopharyngeal carcinoma who underwent laryngectomy and neck dissection at our institution. The MRI and CT findings were compared with the surgical findings and postoperative pathology. In 191 patients, the macro- and microscopic margins of the surgical specimens were tumor-free. In the remaining 6 patients, prevertebral space involvement was observed intraoperatively. MRI predicted the absence of prevertebral fascia invasion in 40 of 42 patients with a negative predictive value of 100% and specificity of 95.2%, while the negative predictive value and specificity of CT were 99.2% and 88.2%, respectively. Preservation of the retropharyngeal fat plane on MRI reliably predicts the absence of prevertebral space fixation in patients with advanced HNSCC.