OBJECTIVES: The long-term risk of child and adolescent psychiatric disorders (PDs) after neonatal invasive group B Streptococcus disease (iGBS) and the modifying factors are poorly understood. METHODS: A population-based matched cohort study, including 1548 infants with iGBS diagnosed during the first 3 months of life from 1997 through 2020 and follow-up until 2022, based on data from Danish national health and administrative registers. A general population comparison cohort of infants without iGBS was randomly sampled and matched 1:10 by sex, the child's birth year and month, and gestational age (n= 15,345). RESULTS: The cumulative incidence proportion (CIP) with 95% confidence intervals (CIs) of any PD was 21.1% (95% CI 18.7-23.7) in infants exposed to iGBS (88% sepsis, 12% meningitis) and 16.2% (95% CI 15.5-17.0) in the comparison cohort. The 18-year CIP of PD was higher for infants exposed to iGBS meningitis compared to iGBS sepsis. The adjusted hazard ratio for PD in infants with iGBS was 1.41 (95% CI 1.23-1.62). CONCLUSIONS: iGBS in early infancy is a risk factor for PDs, especially iGBS meningitis. Premature birth, gestational diabetes, and low maternal education increased the risk of any PD further. SUMMARY: Group B Streptococcus (Streptococcus agalactiae) remains the leading cause of neonatal, invasive disease and is associated with high mortality and risk of neurodevelopmental impairments. Scarce data exist regarding the long-term risk of psychiatric disorders following invasive group B Streptococcus disease in early infancy (iGBS), especially following sepsis. In this Danish national cohort study, we investigated the association of iGBS and the risk of psychiatric disorders until adolescence and early adulthood. We investigated to which extent premature birth, sex (at birth), maternal socioeconomic position or gestational diabetes were modifying the association between iGBS and psychiatric disorders. Our study found an increased risk of psychiatric disorders persisting into adolescence following iGBS, including both meningitis and sepsis. Specific psychiatric disorders with elevated risk included anxiety, obsessive-compulsive disorder, autism, and attention deficit hyperactivity disorder. Premature birth, maternal gestational diabetes, and low maternal education further increased the risk of any psychiatric disorders, whereas the child's sex did not.