Stiff-person syndrome (SPS) is a rare disorder with an estimated prevalence in the general population of 1–2 cases/1,000,000. It is 2–3 times more common in females, with symptom onset at the age of 20–50 years in most cases. Although stiff-person syndrome is associated with antibodies against glutamic acid decarboxylase and amphiphysin, their presence is not necessary for the diagnosis. The treatment should be multidirectional and include immunomodulation, symptomatic treatment as well as monitoring and treatment of overlapping autoimmune, and surgery. We present a case report of a patient diagnosed with stiff-person syndrome overlapping with axonal and demyelinating sensorimotor polyneuropathy. The diagnostic workup indicated diabetes-related polyneuropathy. About 30% of patients diagnosed with stiff-person syndrome also have diabetes. Polyneuropathy alone is rarely reported to overlap with the disorder. In our opinion, polyneuropathy may have a beneficial effect on the clinical presentation of stiff-person syndrome.