CBS first reported the agreement.
The person told the Associated Press that the deal was subject to a physical. The person spoke on condition of anonymity because the contract was not yet complete.
Detroit boldly stepped up in the Fielder sweepstakes after the recent knee injury to star Victor Martinez. A week ago, the Tigers announced that the productive designated hitter could miss the entire season after tearing his left ACL.
The Tigers won their division by 15 games before losing in the AL championship series to Texas. Adding Fielder gives the Tigers two of the game's premier sluggers, pairing him with Miguel Cabrera.
The move also keeps Fielder's name in the Tigers' family. His father, Cecil, became a big league star when he returned to the majors from Japan and hit 51 home runs with Detroit in 1990. Cecil played with the Tigers into the 1996 season.
Several teams had shown interest this winter in the 27-year-old Fielder, who had spent his entire career with Milwaukee. He visited Texas, and the Washington Nationals also got involved in the discussions.
The beefy slugger hit .299 with 38 home runs and 120 RBIs last season. He is a three-time All-Star and was the MVP of last year's event in Phoenix.
Fielder has averaged 40 homers and 113 RBIs over the past five years. He's also been among the most durable players in the majors, appearing in at least 157 games in each of the last six seasons.
The deal is only the fourth $200 million contract in baseball history, following Alex Rodriguez's $275 million, 10-year contract with the New York Yankees, A-Rod's $252 million, 10-year deal with Texas and Albert Pujols' $240 million, 10-year contract last month with the Los Angeles Angels.
Detroit general manager Dave Dombrowski said last week he felt finding a replacement for Martinez was a short-term problem, but he left himself some wriggle room, saying it depended who the player was.
Acquiring Fielder opens all sorts of possibilities, such as moving Cabrera to third base or having one of the two sluggers be the designated hitter.