The incident happened Monday at about 5:38 p.m. at Washington Park beach, police said.
The child, who was wearing red shorts, was seen going into the water on the east side of the catwalk, south of the lighthouse.
Witnesses told police they say the child disappear in the water.
A search was underway for the child overnight.
The Michigan City Fire Department said dive operations were suspended due to hazardous conditions on Monday.
Three divers with the fire department were injured in the search. They were taken to an area hospital for evaluation.
An Indiana Dept. of Natural Resources diver was also injured and was treated at the scene.
All divers were expected to be OK.
"Strong currents, changing wave action, and limited visibility beneath the water can quickly overwhelm even experienced swimmers and rescuers," the fire department said. "We urge everyone to pay close attention."
No other information has been made available.
This is a developing story; check back for updates.