Thome was traded from the Philadelphia Phillies on Saturday for two young minor leaguers -- right-hander Kyle Simon and catcher Gabriel Lino.
The Orioles added Thome to their 25-man roster Sunday and also recalled right-handed reliever Miguel Gonzalez from Triple-A Norfolk.
Thome was inserted in the starting lineup against Cleveland on Sunday, batting fifth.
The 41-year-old Thome is a five-time All-Star. His 609 homers are tied with Sammy Sosa for seventh place on the career list, and his 1,689 RBIs over a 22-year career are 25th all-time.
Thome will move right into the designated hitter role with the Orioles, who have been a surprising contender in the AL East. Out for much of the season with a lower back strain, Thome didn't figure to see much time in the field for the slumping Phillies. He also had struggled as a pinch hitter.
"There's a part of me that's happy to go and get a chance to play a little bit. Baltimore is having a nice year and it will be exciting to go over there and try to help those guys win and get some at-bats. I'm excited about that, for sure," Thome said.
"There's always emotions when you're traded because you have so much respect for the Phillies organization," he said.
In 30 games for the Phillies this season, Thome hit .242 with five home runs and 15 RBIs. He missed more than a month with the back problem and the Phillies didn't want to risk having him get hurt again.
"I think the disappointing thing for me is that I wasn't able to play as much first base as I thought," he said.
As a DH, Thome can purely focus on hitting.
"We felt Thome's presence and our need on the major league team were worth the value we have to give up in a trade," said Dan Duquette, the Orioles executive vice president of baseball operations. "We like both young players. They're both in A ball. We needed some help to bolster our lineup."
Thome appears to be the perfect fit.
"Jim Thome's a real pro, and he specializes in hitting home runs and getting on base," Duquette said. "He's done that consistently his whole career. Beyond that, he's a terrific teammate and a steady presence. He's a veteran, he's been around the league. He's been in the pennant race and he's been in the playoffs. We like all those things that he brings to our team.
"I think he gives us a little bit more depth to our lineup with the injuries we've had, the left-handed hitters in our lineup. I think his presence in the lineup will make the opposing pitchers work harder. Because he gets on base and hits home runs and can hit the ball out of any part of this ballpark, we think he'll be an asset to the team."
The Orioles got a good look at Thome earlier this month when he went 6 for 13 with four RBIs in a three-game series at Camden Yards. He also got his first home run and RBI of the year that weekend.
Thome has done quite well in Baltimore over the years. In 67 career games at Camden Yards, he's hit 18 home runs -- one every 13.6 at-bats -- with 45 RBIs. Of the 65 homers to land on distant Eutaw Street in two decades, Thome has launched two of them.
"I think everybody in baseball has been a fan of Jim, the way he carries himself and conducts himself," Orioles manager Buck Showalter said "The most important thing is this is a guy that can help us with some things between the lines and outside the lines. If you have an opportunity to add him to the mix, you always want to try to do it."
Phillies manager Charlie Manuel told him about the trade after a 3-2 loss at Miami in which Thome didn't play. At the time, the Orioles were still playing in an 11-5 loss to Cleveland.
"I think it's going to give Jim a chance to get more at-bats. And at the end of this year, it will give him a chance to see where he's at and what he wants to do," Manuel said.
"Yes, it's difficult for me to see him go. But at the same time, you look at it from the aspect of what he can do. If he sits here on the bench, it's hard for him to keep his timing," he said.
Baltimore will assume the rest of the one-year, $1.25 million contract he signed with Philadelphia in November 2011.
The Orioles have scored more than three runs in two of their last 12 games and is averaging six hits over the last 14 contests. They have been struggling to produce with runners in scoring position, going 11 for 82 in that situation over a 14-game span.
"I think it's bittersweet, to be honest. Jim's a great teammate and I think we're all sorry to see him go but we're all excited that he'll have the opportunity to get at-bats daily. He can still do a lot of damage," Phillies second baseman Chase Utley said.
Echoed Phillies pitcher Cole Hamels: "He's one of the best guys in baseball and probably one of the most positive influences you could ever have just because he's been around the game."
"He's going to go to the Hall of Fame. It's not every day you get to play with a Hall of Famer," he said.
Marlins manager Ozzie Guillen had Thome on the team with the White Sox.
"The Orioles just picked up a great baseball player, an unbelievable human being, and a great teammate," Guillen said. "I think that's better for him, get more at-bats, play in the American League where he can DH. It will also help the people in the clubhouse because of how good he is."
Simon, 21, was 2-8, 3.96 ERA in 14 starts for Class A Frederick.
Lino, 19, hit .218 four homers and 18 RBIs for Class A Delmarva.
Information from The Associated Press was used in this report.