A 23-year-old previously healthy male was referred to the clinical neurophysiology unit due to a relatively fast-onset paralysis of muscles of the anterior right leg. Electroneuromyography (ENMG) revealed a total denervation of the muscles innervated by the deep peroneal nerve, diminished sensory response of the superficial peroneal nerve, and partial denervation of the peroneus longus muscle. Ultrasound and magnetic resonance imaging (MRI) revealed a large fluid collection inside the common peroneal nerve, primarily suspected to be an intraneural ganglion cyst. The cyst was surgically excised, and the function of the muscles innervated by the peroneal nerve was recovering at the control ENMG 6 months later. We describe a case of a large intraneural ganglion cyst of the peroneal nerve in an otherwise healthy young male, diagnosis by ENMG, ultrasound, and MRI, as well as subsequent operative treatment. This report demonstrates the utility of nerve ultrasound in differentiating between different causes of peroneal nerve dysfunction.