Introduction: Stiff person syndrome (SPS) is an uncommon and debilitating condition marked by increasing muscle stiffness, spasm, and rigidity. It classically presents with truncal and appendicular muscle spasms associated with lumbar hyper lordosis. It may be caused by autoimmune mechanisms, paraneoplastic factors, or have an unknown origin. This condition can occur alongside other autoimmune diseases, including type 1 diabetes mellitus, pernicious anemia, vitiligo, and Hashimoto's thyroiditis.

Up to 80% of patients have positive anti glutamic acid decarboxylase (GAD) antibody. Few individuals are GAD negative.

Case report: We present a 32-year-old male who presented with intermittent tonic spasm and was misdiagnosed as seizures and was treated for the same. He was seen in our hospital during one of the episodes and was suspected of having stiff person syndrome (SPS). His anti-GAD antibody and paraneoplastic workup were negative. His electromyogram (EMG) showed continuous spontaneous activity in both agonist and antagonist muscles, confirming the diagnosis of possible SPS -seronegative. He was started on intravenous immunoglobulin and benzodiazepines and showed significant improvement.