The Cincinnati Fire Department said the collapse onto southbound interstate 75 happened near Hopple Street north of downtown at about 10:30 p.m. Monday.
Police Chief Jeff Blackwell called it a workplace accident, saying crews were taking down the old bridge when something went "terribly wrong."
"The big-rig driver is very lucky; in a matter of seconds his fate would have probably been different," Blackwell said.
The driver was taken to the University of Cincinnati Medical Center with minor injuries.
Fire officials said the tractor-trailer struck the collapsed section as the bridge hit the ground. They said the construction worker was killed in the collapse. His name was not immediately released.
Blackwell said the city will work with the Department of Transportation to figure out what happened. He said several hundred tons of concrete had to be removed from the road.
The southbound lanes of I-75 would be closed for at least 48 hours, officials said.
A witness told WLWT5 that he heard "a real big boom" and then a couple of seconds later he saw police cars rushing to the scene.