OBJECTIVES: Tyrosinemia type III is an extremely rare autosomal recessive disorder of tyrosine metabolism caused by mutations in the HPD gene, which encodes 4-hydroxyphenylpyruvate dioxygenase (HPPD). Wolff-Parkinson-White (WPW) syndrome is a congenital cardiac conduction disorder characterized by the presence of an accessory atrioventricular pathway. While each condition is rare in isolation, their coexistence has not been previously reported. CASE PRESENTATION: We present a unique case of a 6-year-old boy with known WPW syndrome who was admitted with ketotic hypoglycemia after prolonged fasting and omission of propranolol doses. Metabolic work-up revealed persistently elevated plasma tyrosine levels. Genetic testing confirmed tyrosinemia type III due to a novel homozygous HPD variant [c.559A>G (p.Asn187Asp)]. The persistence of the WPW pattern despite decreased plasma tyrosine levels suggests that there is no direct causal relationship. He was also diagnosed with attention-deficit/hyperactivity disorder, specific learning disorder, and borderline intellectual functioning. CONCLUSIONS: This case highlights the importance of metabolic evaluation in pediatric patients presenting with unexplained hypoglycemia, particularly in the presence of pre-existing cardiac disorders.