Cardiff and Vale health board said admissions to the University Hospital of Wales will go to a different ward or a different hospital.
The board has not confirmed what the infections are.
Babies already being treated on the ward will remain there, so it has not closed completely.
Ruth Walker, the health board's executive nurse director, said: "In order to protect very vulnerable babies we will not be taking any planned or routine admissions over the coming days."
The neonatal intensive care unit at a Cardiff hospital has stopped taking admissions due to a "cluster of infections".