Willingham, 32, is a career .262 hitter with 132 home runs for the Marlins, Nationals and Athletics. He batted .246 with a career-high 29 homers and 98 RBIs for Oakland in 2011.
Cleveland, Tampa Bay, Colorado and Boston are among the other clubs that expressed interest in Willingham during the free agent process.
Willingham's decision to sign with Minnesota will probably spell the end for incumbent outfielders Michael Cuddyer and Jason Kubel in Minnesota. The Twins selected Cuddyer in the first round of the 1997 first-year player draft, and he has spent his entire 10-year MLB career in a Minnesota uniform.
Jerry Crasnick is a senior baseball writer for ESPN.com.