Introduction. Group B streptococcus (GBS) is a leading cause of invasive neonatal infections. These have been divided into early-onset disease (EOD; <7 days) and late-onset disease (LOD; 7–89 days), with different GBS clonal complexes (CCs) associated with different disease presentations. Hypothesis. Different GBS CCs are associated with timing of infection (EOD or LOD) and clinical presentation (sepsis, meningitis or pneumonia). Aim. To study infant GBS infections in Iceland from 1975 to 2019. Are specific GBS CCs related to disease presentation? Is CC17 overrepresented in infant GBS infections in Iceland? Methodology. All culture-confirmed invasive GBS infections in infants (<90 days) in Iceland from 1975 to 2019 were included. Clinical information was gathered from medical records. Results. A total of 127 invasive GBS infections in infants were diagnosed, but 105 infants were included in the study. Of these, 56 had EOD and 49 had LOD. The incidence of GBS infections declined from 2000 onwards but increased again at the end of the study period. Furthermore, there was a significant increase in LOD over the study period (P=0.0001). The most common presenting symptoms were respiratory difficulties and fever and the most common presentation was sepsis alone. Approximately one-third of the cases were caused by GBS CC17 of serotype III with surface protein RIB and pili PI-1+PI-2b or PI-2b. CC17 was significantly associated with LOD (P<0.001). Conclusion. CC17 is a major cause of GBS infection in infants in Iceland. This clone is associated with LOD, which has been increasing in incidence. Because intrapartum antibiotic prophylaxis only prevents EOD, it is important to continue the development of a GBS vaccine in order to prevent LOD infections.