Financial terms were not announced by the Rangers (http://texas.rangers.mlb.com) but Major League Baseball's official website (www.mlb.com) said the deal was worth $96 million.
That would make the contract the second-largest ever agreed to by the Rangers. Current New York Yankee Alex Rodriguez signed a 10-year, $252 million deal in 2000.
Beltre, 31, a two-time Golden Glove Award winner and 2010 All-Star, hit .321 with 102 runs batted in during his only season in Boston. He previously played five seasons with Seattle Mariners.
Young is expected to become the Rangers' designated hitter with the club not re-signing Vladimir Guerrero.
(Reporting by Gene Cherry in Raleigh, North Carolina; editing by Steve Ginsburg)