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Aim: Clusters of group B Streptococcus (GBS) infections in neonatal intensive care units (NICU) are poorly documented. We aimed to assess GBS cross‐transmission during an outbreak of GBS sepsis. Methods: The study was carried out between October and November 2021 in a French University Hospital. Neonatal intensive care unit (NICU) patients with GBS sepsis were included. Clinical data were retrieved from electronic patient records. Group B Streptococcus isolates were characterized at the molecular level using capsular genotyping and whole‐genome sequencing (WGS). Results: The outbreak of GBS sepsis affected three very preterm neonates with a gestational age of less than 26 weeks, including one recurrent male index case aged 26 days, and two female secondary cases aged 5 and 17 days. The microbiological investigation identified a GBS isolate of capsular type III and Sequence Type 17 as responsible for the four infectious episodes. Whole‐genome sequencing confirmed the identity between the isolates. The outbreak and the results of the microbiological investigations led to an immediate reinforcement of hygiene measures. Conclusion: Clustered cases of GBS infections in NICU and horizontal transmission of the hypervirulent GBS Sequence Type 17 are likely underestimated. Prospective investigation of all nosocomial cases using WGS should contribute to improving vigilance regarding GBS cross‐transmission and infection prevention. [ABSTRACT FROM AUTHOR]