Sandoval, 28, won three World Series championships with the Giants, but he chose to sign a five-year, $95 million contract with the Boston Red Sox after winning the 2014 World Series.
He said his decision to leave the Giants was largely made after contract negotiations broke down during spring training last year.
"I knew early in spring training last year I was going to leave," Sandoval said. "They didn't respect my agent. Contract talks, everything. The way Brian Sabean (Giants general manager) talked to my agent."
Sandoval said that he signed with the Red Sox despite being offered more money from the Giants.
Last season, his final one in San Francisco, Sandoval batted .279 in 157 games with 16 home runs and 73 RBI. During his seven seasons with the Giants, he batted .294 while making two All-Star teams.
Sandoval will make his debut with the Red Sox on Monday, April 6 against the Philadelphia Phillies.
- Jack Jorgensen