Boston is in contention for the AL East and wild-card spots, and had been playing Nick Green at shortstop.
"Boston really wanted [Gonzalez]," Reds manager Dusty Baker said. "You hate to see him go. This is a big trade for him. He's in the heat of the pennant race, and they liked him when he was there before, big time."
Gonzalez will join the Red Sox for Saturday night's game in Texas. He hurried out of the Cincinnati clubhouse without commenting.
He is hitting .210 with three home runs and 26 RBIs this season.
The 32-year-old Gonzalez played for Boston in 2006, hitting .255 with 50 RBIs and setting a team record for shortstops with a .985 fielding percentage. He then signed a three-year contract with the Reds, a deal that included a mutual option for the 2010 season.
Gonzalez is a career .246 hitter with 109 homers and 506 RBIs during 11 seasons with Florida, Boston and Cincinnati. He played for the World Series champion Marlins in 2003, and won Game 4 with a home run in the bottom of the 12th inning against the New York Yankees.
Gonzalez missed the 2008 season after having surgery on his left knee.
Rookie Paul Janish took Gonzalez's place in Cincinnati's starting lineup, at shortstop and in the No. 2 spot in the batting order, for Friday night's game against Washington.
"Me and Alex had a great relationship," said Janish, who went into the game hitting .216 in 42 games, including 22 starts and two relief pitching appearances. "Trades are the nature of the game. I wish him a lot of success, but I'm excited to get the opportunity to play."
The 23-year-old Negron hit .264 with 20 stolen bases for Salem in the Carolina League.
The Reds also promoted first baseman Kevin Barker from Triple-A Louisville. Barker, 34, hit .284 with 22 homers and 68 RBIs in the minors, and has had major league stints with Milwaukee, San Diego and Toronto. He has a major league career average of .245 with six home runs and 33 RBI in 97 games.
"I was very surprised," he said about learning that the Reds had purchased his contract.
Barker will serve primarily as a left-handed pinch-hitter, Baker said, especially with left-handed hitting Joey Votto in front of him.
"I'll play wherever they want me to play," Barker said. "It's just an honor and a privilege to be here."