Garciaparra's uncle Victor Garciaparra told the Boston Herald that Nomar was in his Charlestown condominium around 10 p.m. Friday when the two men heard a scream and a splash.
As soon as the All-Star shortstop and two-time American League batting champ ran out the door to help the woman, her friend also fell in, hitting her head on the pier, Victor Garciaparra said.
Victor Garciaparra, who oversees his nephew's business and charitable ventures, jumped from the balcony to the water 20 feet below.
"I swam towards them and by the time I reached them, Nomar was already there holding the girls up," Victor told the newspaper. "But he couldn't get them up without help."
One woman had a large lump on her head and appeared to be unconscious, he said. When she came to, the first thing she said was: "Are you Nomar?" Victor Garciaparra said.
The two men pulled the women from the water. Their husbands arrived and took them to the hospital before the Garciaparras got their names, he said.
Johnny O'Hara of Natick, Mass., witnessed the incident from his boat.
"A bunch of us came running over and sure enough, pulling the two girls from the water was Nomar," O'Hara told the Boston Herald. "It was crazy. Nomar was like jumping over walls to get to the girls and the other guy leaped off the balcony. It was unbelievable."
Nomar Garciaparra was unavailable for comment, his uncle said. Garciaparra played for the Boston Red Sox from 1996 until he was traded to the Chicago Cubs in July 2004. He batted .283 in an injury-shortened season for the Cubs.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.