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Background Stiff-person spectrum disorder (SPSD) is a rare autoimmune neurological disorder characterised by progressive rigidity, muscle co‐contractions and spasms; 5%–10% are paraneoplastic. Few infection-related SPSD cases have been reported.Case presentation We describe a 56-year-old woman who developed SPSD after herpes zoster. Eight days after rash onset, she experienced persistent lower-limb tonic spasms with co‐contractions, autonomic symptoms and elevated anti-glutamic acid decarboxylase antibodies in serum and cerebrospinal fluid. Immunotherapy with repeated high-dose intravenous immunoglobulin and methylprednisolone pulses, along with continuous benzodiazepines and epidural analgesia, led to gradual improvement over 3 months.Conclusions Pain from herpes zoster may have amplified muscle co-contractions via withdrawal and crossed extension reflexes, exacerbating impaired reciprocal inhibition in SPSD. Clinicians should consider optimal pain control in addition to immunotherapy for SPSD triggered by infection.