Emergency crews responded to Randall's Island on Saturday afternoon for reports that a young boy fell into the waters near Field 7.
The U.S. Coast Guard dispatched teams to join the ongoing search along the island's shoreline. Officials have been searching in the Harlem River since 3 p.m.
Witnesses say a number of people nearby jumped into the river to try and help search for the boy.
Law enforcement sources told News 4 the 5-year-old boy fell into the river after climbing a tree.
This story is developing.