Glutathione synthetase (GSS) deficiency is a rare disorder of glutathione metabolism with varying clinical severity. Patients may present with haemolytic anaemia alone or together with acidosis and central nervous system impairment. Diagnosis is made by clinical presentation and detection of elevated concentrations of 5-oxoproline in urine and low GSS activity in erythrocytes or cultured skin fibroblasts. Diagnosis can be confirmed by mutational analysis. Treatment consists of the correction of acidosis, blood transfusion, and supplementation with antioxidants. The most important determinants for outcome and survival in patients with GSS deficiency are early diagnosis and early initiation of treatment. The case of a newborn with GSS deficiency diagnosed by tandem mass spectrometry (MS/MS)-based newborn screening is described. After onset of clinical symptoms on the 2nd day of life, expanded newborn screening revealed normal results for all disorders included in the German screening programme; however, selective MS/MS screening revealed a >10-fold elevation of 5-oxoproline in dried blood, leading to the presumptive diagnosis of GSS deficiency by the 5th day of life. Diagnosis was later confirmed by detection of markedly reduced glutathione concentration in erythrocytes and mutational analysis of the GSS gene. Presently, GSS deficiency is not included in newborn screening programmes in Europe. As outcome depends significantly on early start of treatment, routine inclusion of this disorder in newborn screening panels should be considered.