The incident occurred in the second innings of a Boston Red Sox versus Oakland Athletics game in the Red Sox' home ground Fenway Park.
Brett Lawrie of the Oakland Athletics was batting when his bat shattered on a swing against left-hander Wade Miley.
The bat snapped as it made contact with the ball sending a large piece flying into the stands where it hit the unnamed woman in the face.
The game was delayed as the fan was stretched out of the stadium with blood covering her face to a local hospital. She appeared to remain conscious but could be heard shrieking.
A fan who witnessed the incident told the Boston Globe: "Brett Lawrie hit the ball and the bat snapped in half near the end of the bat."
"It hit on the forehead to the top of the head… it was a blunt trauma and it was a lot of blood. I don't think I've ever seen that much blood."
WARNING – GRAPHIC IMAGES
A women has left Fenway in a stretcher after being struck by a broken bat. Thoughts & prayers. pic.twitter.com/GBLEAsMos9
— Only In Boston (@OnlyInBOS) June 6, 2015
A shard from Brett Lawrie's broken bat struck fan sitting in third-base boxes. Bleeding pretty profusely. Other fan offers shirt to medics
— Gordon Edes (@GordonEdes) June 5, 2015
The woman was attending the game wither husband and son, who had to be consoled by a police officer and medical staff.
A policeman was holding a young child and shielding his or her eyes. Fans in the area trying to help. Lots of blood. Awful scene.
— Pete Abraham (@PeteAbe) June 5, 2015
According to Boston police, the fan was taken to Beth Israel Deaconess Medical Center, where she was receiving treatment for her injuries. The victim is thought to be in a stable condition now.
The game continued after the woman had been carried off.
Major League Baseball uses bats made of from ash wood and maple wood. Maple bats have been proved divisive in recent seasons because they tend to break.
After Friday's accident, the scrutiny of the controversial bats is likely to intensify.
The game finished with Boston winning 4-2, ending Oakland's four-game winning streak.