Background: Stiff person syndrome (SPS) is a rare disorder presenting with progressive stiffness and spasms of the musculature of the trunk and limbs. SPS is reported very rarely in children and adolescents, with 5 cases over 25 years in a recent 99 patient cohort. Methods: Case Study Results: Herein we report a 15 year old female, presenting with acute onset of rapidly progressive spasticity of the lower extremities. Initial exam was remarkable for markedly limited left knee range of motion, in addition to asymmetrical knee spastic catch and hyper-reflexia. EMG revealed almost continuous motor unit activity which dissipated with voluntary muscle contraction. Diagnosis was confirmed by high titres of glutamate decarboxylase (GAD65) antibodies >25,000 units/ml. The patient was initially treated with IVIG, baclofen, and diazepam followed by IV methylprednisolone, with mild subjective improvements. One day following the first rituximab treatment, she achieved spontaneous knee flexion and regained the ability to ambulate independently. There is a residual spastic catch at the knees. Conclusions: This case highlights that SPS, albeit extremely rare, should be considered in the differential diagnosis of acquired spasticity in children. Also noteworthy is the relatively rapid resumption of function with aggressive immunomodulatory treatments in this historically devastating disorder.