Ludwick, who is hitting .238 with 11 home runs and a team-best 64 RBIs, was shipped to the Pirates for a player to be named later.
Ludwick, who turned 33 on July 13 and can play left and right field, will be a free agent following the season. He's making $6.775 million this season.
Ludwick was in the original starting lineup for Sunday's game against the Rockies, but he was later scratched.
"I had everything on, ready to go," Ludwick said, smiling.
Instead, Ludwick found himself heading back to the National League Central, where he spent parts of four seasons with the Cardinals from 2007-10.
"I'm going to another team that has a chance to make the playoffs," Ludwick said. "I'm going back to the NL Central, a division I'm familiar with and a ballpark where I've done pretty well."
The Padres obtained Ludwick on July 31 of last season as part of a three-team and four-player deal. The hope was that Ludwick would give the Padres another middle-of-the-order bat for the pennant run and to ease the burden on Adrian Gonzalez.
Ludwick hit .211 with six home runs and 26 RBIs after the trade, and admitted in Spring Training that he was pressing too much and that his swing got "too loopy."
"Unfortunately, I sort of stunk up the joint last year," he said.
While only with the Padres for one calendar year, Ludwick certainly made an impression on his teammates.
"I think other than David Eckstein, he's [Ludwick] as professional a teammate as I have been around," said Padres catcher Nick Hundley. "His ability to come to the park every day and no matter whether he's going or if he's going bad ... he's still a great teammate.
"He never complains, he never pouts, he goes out there and plays hard and he plays hurt. There are just a lot of things he brings to the table as far as being a great clubhouse guy. Those are the kind of guys you want on your team."
San Diego general manager Jed Hoyer said that trading Ludwick "clears up a spot for a young player to get some at-bats." On Sunday, that player was Kyle Blanks, who was added to the lineup when Ludwick was scratched.
Hoyer also indicated that outfielder Aaron Cunningham will be at PETCO Park on Monday for the start of a series against the Dodgers. Cunningham is hitting .329 with nine home runs and 63 RBIs this season with Triple-A Tucson.
Oddly enough, the Padres open a three-game series in Pittsburgh on Friday, which will give Ludwick a chance to face his former teammates.
"We had a great group here, but it's unfortunate we didn't win," Ludwick said of his time with the Padres. "Bud Black ... I don't know if there's a better manager to play for. I will never say anything bad about this organization."