The fan, said by the Pirates to be alert when she was wheeled out, was finding her seat with her back to the field of play as Cubs shortstop Starlin Castro fouled a ball straight back.
The ball hit the netting as well as the side of the woman's head, which was close enough for contact.
She immediately put her hand to her head and collapsed to the ground.
After initially receiving treatment at the park, the woman was taken to a hospital for additional treatment.
"She was conscious and alert at the time of transport, graciously thanking her caregivers while giving a 'thumbs up' gesture to nearby fans," the team said in a statement. "Our thoughts and prayers are with her."
After the game, Cubs manager Joe Maddon reacted with dismay to the incident.
"[Outfielder]Chris Coghlan thought the ball hit the wall," Maddon said. "That's how loud it was. The umpire said the same thing. It was that really hard, cracking sound.
"The fact nobody is getting up, and there are ice bags, we had no idea what was going on. Couple guys saw it and thought it was horrid."
Fan injured after being struck in head by baseball
The Baseball Tonight crew discusses fan safety inside a ballpark after a fan was struck in the head by a foul ball during Monday's Cubs-Pirates game.