Officers saw the 8-month-old unconscious and unresponsive lying on the embankment on the edge of the East River near Dover Street and South Street under the Brooklyn Bridge. An eyewitness said the baby was only wearing diapers.
Police say someone waved them down around 4 p.m. Sunday. A man had jumped into the river to perform CPR on the baby.
The boy was transported to the hospital where he was pronounced dead.
No parents or guardians were present at the scene.
The eyewitness also said it did not appear that the baby fell in from the shore.
No signs of trauma were found on the baby. The Medical Examiner is working to determine the cause of death.