On Friday, he moved back in.
Morales, who turned down larger offers from Seattle last winter and eventually signed with Minnesota as a free agent, was reacquired in a trade Thursday.
"It caught me by surprise," Morales said through an interpreter. "It sort of came together quickly. It's a surprise, and I'm glad to be here."
Wearing jersey No. 21, Morales batted fourth as the designated hitter in a 2-1, 10-inning loss to the Baltimore Orioles on Friday night. He went 0-for-3 but hit a sacrifice fly in the bottom of the sixth that scored Robinson Cano and tied the game at 1.
The Mariners traded minor league pitcher Stephen Pryor to get the 31-year-old Morales. They hope he can bolster their slumping lineup.
"I just want him to be a stabilizer, drive in runs, good at-bats, productive at-bats," Mariners manager Lloyd McClendon said. "I can tell you this, he's not going to be overwhelmed by any situation. He'll be a nice piece to have in our lineup."
Morales hit 23 home runs with 80 RBIs last year in his one season with the Mariners. He then turned down a $14.1 million qualifying offer and became a free agent, and also rejected other overtures this year from Seattle.
Morales finally decided to sign with the Twins on June 8.
"I made the decision last year to be a 100 percent free agent and I don't feel bad about the decision," said Morales, who has been represented by agent Scott Boras.
Morales did not deliver what the Twins had hoped. After missing all of spring training and two months of the season, he was immediately thrown into the lineup and hit just .234 with one home run and 18 RBIs in 39 games.
Morales said his time with Minnesota served as a kind of spring training. He believes he's rounding into shape.
"I'm getting a few more at-bats under my belt," he said. "I've been playing better lately and swinging the bat with a little more authority."
Morales is also getting a chance to play in more meaningful games. The Mariners are in the middle of the American League wild-card race while Minnesota is last in the AL Central.
In his first game since rejoining the Mariners, Morales batted after Cano and ahead of fellow All-Star Kyle Seager.
"It's huge," Seager said of the acquisition. "You have another impact bat in the middle of your order."
McClendon said Morales' arrival will give him more flexibility in the lineup. He's considering switching Seager from the No. 5 spot to No. 2, ahead of Cano and Morales.
"I'm comfortable wherever," Seager said. "Once you're in the batter's box, the order doesn't really matter. Obviously, hitting in front of a guy like Cano, you'll be pitched a little differently. At same time, it's still kind of all the same."
The Mariners rank 14th in the AL with a .244 batting average and are ninth in home runs.
"It's good when organization feels that way about you," Morales said of his return. "I felt very comfortable with my time here last year and with this team, and am going to feel even more comfortable this year."
The Associated Press contributed to this report.