Pablo Sandoval agreed to a five-year deal with the Red Sox worth a reported $95 million.
The deal also includes a sixth-year club option worth $17 million, with a $5 million buyout, according to a report.
The Giants' final offer for Sandoval, vice president and assistant GM Bobby Evans revealed on Yahoo SportsTalk Live on Monday, was a five-year, $95 million contract that included a club option for a sixth year and a willingness to go up.
Sandoval was asked the following question at his introductory press conference in Boston on Tuesday:
"By most accounts, the offers between the Red Sox and the Giants were nearly identical. Given the success that you had in San Francisco, why did you choose coming here rather than staying in a place that you had enjoyed?
"It was a tough decision for me," Sandoval said. "Took me a long time to be sure I made the right decision. The Giants gave me an opportunity to be in the big leagues, opened the door, teach me how to respect the game.
"The Giants fans, one of the best ... But I wanted a new challenge ... I need a new challenge."
San Francisco signed Sandoval as an amateur free agent out of Venezuela back in 2003.
He made his big-league debut on Aug. 14, 2008 at the age of 22 and over six full seasons with the Giants, registered a .292 batting average with 103 home runs and 438 RBI.
He was named the 2012 World Series MVP, was an All-Star twice and his .344 career batting average is the highest in postseason history (minimum 150 plate appearances).
His .426 career batting average in the World Series is second in MLB history (minimum 50 plate appearances) to his new teammate David Ortiz (.455).
Is it weird to be wearing a different hat and a different uniform?
"Feels great, man," Sandoval answered. "I know that I had a great career in San Francisco but I'm gonna have a new one here. New challenge.
"I'm gonna start like zero ... Feels great to be in a Boston Red Sox uniform right now."