The tragedy occurred at approximately 4pm at the Lyreen River close to the Kilcock Road in Maynooth, Co Kildare.
The three teenagers, a girl and two boys, were playing by the river when it is believed they got into difficulty.
A Garda spokesperson said one of the boys managed to get out of the river and called the emergency services.
The three teenagers were rushed to Our Lady’s Hospital for Sick Children in Crumlin, Dublin.
However, the 13-year-old boy passed away some time later.
The other two teenagers are not believed to be in a serious condition following the incident.