The deal, confirmed on Sunday by MLB.com, was pending a physical, but was completed, meaning Inge will be in Boston for the club's three-game series opener against the Red Sox. He will bat eighth and play third base.
Once in uniform, it's safe to assume his place will be the everyday third-base role, following an early round of musical chairs at the position that saw a lack of production from Josh Donaldson, Eric Sogard and the recently acquired Luke Hughes. Combined, A's third basemen had a .127 average with a .374 OPS, two home runs and six RBIs entering the day.
The A's designated Hughes for assignment to make room for Inge on the 25-man roster.
Inge, who will be 35 next month, has endured his own struggles in April, compiling just two hits in 20 at-bats spanning nine games with Detroit, leading to his release from the only organization he's known Thursday. But he's considered an excellent defender, and his past production at the plate, though not overwhelming, represents an upgrade for Oakland's sluggish offense.
The Tigers are on the hook for his $5.5 million salary, while the A's will only be responsible for the pro-rated Major League minimum.
His most productive year came during Detroit's 2006 American League championship season, when he hit .253 with 27 home runs and 83 RBIs in 159 games. He was also an All-Star in 2009, when he tallied 21 homers in the first half. But he's hit just 23 since and, overall, owns a .234 career average and .691 OPS.
"It'll be good to get a fresh start with another team," Inge said last week. "I don't have any doubts. I'm a team player. I'm not selfish. I'm not going to talk bad about anyone. I'm going to go out and do my job."
The infielder won a part-time job at second base in Spring Training, but began the season on the disabled list with a strained left groin. He made his debut on April 14 and saw most of his starts at second base against left-handed starters, striking out six times without a walk.
Inge represents the 12th third baseman Oakland has employed since Eric Chavez played his last game there for the A's in 2009. Aside from Sizemore, who suffered a season-ending knee injury on the first day of spring workouts, and Donaldson, Sogard and Hughes, the others who have endured stints at the hot corner are Conor Jackson, Kevin Kouzmanoff, Andy LaRoche, Adam Rosales, Akinori Iwamura, Jack Hannahan and Adam Kennedy.
Hughes was 1-for-13 since joining the A's last week.