Background. Nipah virus (NiV) causes severe diseases in humans with a high case fatality rate. The primary risk factors for NiV infection in Bangladesh are drinking raw date palm sap (DPS) contaminated with Pteropus fruit bat secretions/excretions or close contact with or exposure to the body fluid of an individual with NiV infection. Methods. During the 2023 NiV outbreak investigation in Bangladesh, the breast milk of a NiV-infected nursing mother was tested by real-time reverse transcriptase polymerase chain reaction (RT-PCR) for detection of NiV-RNA. The newborn was also tested as a suspected NiV-infected subject. Results. NiV, specifically NiV RNA, was detected in the breast milk sample. Through the investigation, it was determined that the mother consumed raw DPS nine days before the delivery. The newborn was also confirmed as NiV positive and had exposure to maternal bodily fluid during delivery, while breastfeeding, and was in prolonged maternal contact during caregiving. Conclusion. By revealing breast milk as a potential source for NiV transmissions, our findings advocate for incorporating breast milk testing into NiV diagnostic protocols for symptomatic mothers. This advancement will broaden our understanding of mother-to-child NiV transmission and pave the way for more effective containment strategies.