The 28-year-old third baseman said he was ready for a new challenge after helping the Giants win three of the last five World Series titles. Sandoval added he wouldn't have returned to San Francisco for a larger offer.
"No. I left more money out there. I didn't let the Giants make the final offer," Sandoval said. "I just wanted to get out and try new things, try a new challenge. I just want to be a part of a new challenge."
Sandoval joins a revamped Boston lineup, which includes fellow free-agent acquisition Hanley Ramirez, that is looking to improve on last year's last-place finish.
"I think they did a great job putting the offense together, trying to get more patient," Sandoval said. "Hanley's doing very good, moving from shortstop to left, to see opportunity to have more guys in there. We can do more damage on offense."
When asked if the Red Sox will win the division, Sandoval responded by saying, "We will."
Cash Kruth is a reporter for MLB.com. Follow him on Twitter at @cashkruth. This story was not subject to the approval of Major League Baseball or its clubs.