IMPORTANCE: Progressive encephalomyelitis with rigidity and myoclonus is characterized by rigidity, painful muscle spasms, hyperekplexia, and brainstem signs. Recently, glycine receptor alpha 1 antibodies have been described in adult patients with progressive encephalomyelitis with rigidity and myoclonus. We describe a pediatric case. OBSERVATIONS: A 14-month-old child developed startle-induced episodes of generalized rigidity and myoclonus, axial hyperextension, and trismus, without impairment of consciousness. Episodes occurred during wakefulness and sleep, lasted seconds, and were accompanied by moaning, tachypnea, and oxygen desaturation. Imaging, cerebrospinal fluid, endocrine, metabolic, and genetic screening findings were normal or negative. She was treated with intravenous steroids and immunoglobulins with resolution of symptoms, but she relapsed weeks later. At this time, episodes were more severe. Glycine receptor alpha 1 antibodies were found in serum (titer of 1:200, later 1:320) and cerebrospinal fluid (titer of 1:2). Treatment was restarted with intravenous steroids and immunoglobulins, with major improvement, and she began treatment with oral steroids. She had 4 milder relapses, with improvement after treatment adjustments. CONCLUSIONS AND RELEVANCE: To our knowledge, this is the first pediatric case of progressive encephalomyelitis with rigidity and myoclonus associated with glycine receptor alpha 1 antibodies, a potentially severe but treatable antibody-mediated neurological disorder.