A Brewers spokesman said the boy was checked out at first aid and was fine.
In the bottom of the fourth, Alcides Escobar's bat shattered when he grounded out to end the inning. A sharp piece of the barrel flew into the Miller Park stands about 10 rows behind the Pirates' on-deck circle, careening off several fans and hitting the boy in the head.
After being tended to by ushers and medical personnel, the boy, holding a baseball, walked up the stairs under his own power and was cheered by other fans in his section.
One day earlier, a fan sustained minor injuries after falling over an outfield railing and onto the field before a Brewers game against the Chicago Cubs.
Many maple bats were banned in the minor leagues this season as part of Major League Baseball's push to stop shattered shards of wood from flying dangerously through the air.
MLB and the union have been extensively studying the issue of broken maple bats since 2008, as splintered barrels wildly helicoptered all over the field and into the stands.
Also, during spring training, Minnesota outfielder Denard Span lined a foul ball that hit his mother sitting in the stands near the Twins' dugout.
Wanda Wilson received some medical attention on the scene and returned to her seat later in the inning with nothing more than a sore chest. But the scare prompted Span to renew his call to extend the netting behind home plate that protects fans from flying objects.