Pedro Martinez was waving from the Boston Duck Tours boat to the throngs lining the Boston side of the Charles River when a ball flew from the crowd, plunked the star pitcher in the forehead and plopped in the water.
Witnesses said Martinez appeared stunned, but he told the Associated Press after the parade: "I have a little headache, but I'm OK."
Dr. Charles Steinberg, executive vice president of public relations for the Red Sox, concurred that Martinez was not seriously hurt.
"He was here at Fenway Park afterward. He seemed fine," Steinberg said.
It was unclear who threw the ball or why. Boston police spokeswoman Nadine Taylor-Miller said the department had no knowledge of the incident and is not investigating.
Taylor-Miller said that throwing a baseball at someone's head could be grounds for an assault and battery charge.
"Why would anyone throw a ball?" she asked. "Were they throwing it for him to catch it? I don't know."
Witnesses near the Longfellow Bridge weren't sure either.
"It just came from the crowd maybe 10-20 yards away," said Amy Kittredge, 24.
"It just kind of flew out of the crowd," Kittredge said. "It was either a softball or a baseball. It looked hard. It hit him right in the head.
"He brought his hands to his head like, 'Holy cow! Did something just hit me in the head?' It looked like it hurt. It looked like it really clunked him. We felt really bad."
Another fan who witnessed the beaning, Jim Averill of Boston, told the Associated Press that the ball was thrown from the river bank "at a high velocity."
"I saw the ball hit him in the head," Averill said. "Derek Lowe tried to grab it. It hit him and it flew up about 15 feet and fell into the water."
The ball then floated down river, Averill said.
A police officer walking along the parade route said that at the end of the parade, several fans on the Cambridge side of the river also threw hats and other Red Sox souvenirs at Martinez, hitting him in the face.
"They wanted autographs," the officer said.