The man, who works for the Chicago Department of Transportation, was cleaning springs that help open and close the bridge when a support gave way.
"It was pretty shocking. Yeah, I was right behind him when he fell. I just kept talking to him, telling him help was on the way. Told him not to move. He was trying to get up," CDOT Supervisor Mike Tracy said.
The Randolph Street Bridge was open for the routine maintenance and the worker fell into a small trench area under it. A Chicago Fire Department special operations unit was called in.
"The first thing, the bridge was up. So there is no way while we are under there working with him," Chicago Fire Department Chief Michael Fox said. It was a tight space, and a water rescue team was called in just in case.
"It was a little hairy. They did a really great job," Fox said.
Traffic was re-routed on Wacker and Canal as firefighters worked to free the worker along the Chicago River.
Workers with the CDOT are doing routine maintenance on river bridges in preparation for the boating season.