The 6-foot-10 Young returns to action as the Padres struggle to retain the NL West lead. He made one start this season before going on the disabled list April 12.
The Padres go into Thursday night's series opener at St. Louis with a half-game lead over San Francisco.
Young is coming off three rehab starts in the minor leagues. On Saturday night, he allowed three hits in 5 2/3 scoreless innings with seven strikeouts and one walk for Class A Lake Elsinore.
The day after that start, Young said he felt good enough to pitch again in the big leagues. Manager Bud Black agreed, making it official on Thursday.
Young had arthroscopic surgery on his right shoulder in August 2009.
He made a sparkling season debut on April 6, allowing one hit through six innings in a 6-3 win at Arizona. But he felt tightness in his shoulder and went on the DL six days later.
Young has had a serious injury each season since 2008, when he was hit in the face by an Albert Pujols line drive at Petco Park.
Young remains the only Padres pitcher to win a postseason game since 1998. With the Padres trailing the Cardinals 2-0 in the 2006 division series, he threw 6 2/3 shutout innings in Game 3 at Busch Stadium as the Padres won 3-1. The Padres were eliminated the next day.
Ryan Ludwick was back in St. Louis for the first time since he was traded to San Diego on July 31 in the three-team swap that moved right-hander Jake Westbrook from the Cleveland Indians to the Cardinals. The right fielder was batting fifth Thursday night in the opener of an important four-game series for the Padres.
Ludwick says he loved his time in St. Louis and was excited to be back. He says he has no hard feelings about the trade.
"It was a first," Ludwick said about finding the visitors' locker room. "A lot of strange feelings, you know, obviously emotional. I loved my time here. There's always going to be a huge place in my heart for St. Louis. I'm definitely excited to be back.
"I think when it comes to trades in the game of baseball, everyone knows it's a business. Things happen and you move on. I really couldn't control the trade," he said.
Information from The Associated Press was used in this report.