An employee picking up trash outside the David L. Lawrence Convention Center heard the woman calling for help Thursday morning, the Pittsburgh Tribune-Review reported.
The rescue was done by two divers from the city paramedic team. They walked almost 300 feet down the flooded tunnel with an inflatable raft and brought the woman out.
The convention center is on the banks of the Allegheny River. The city was hit by heavy rain Wednesday night.
"She must have been sleeping in there last night and got in there before the river rose," said Mark Johnson, the center's facilities director. "As the river rose, she couldn't get out the way that she got in."