The 6-foot-5 pitcher was recalled from Triple-A Iowa on Tuesday, replacing righty Jose Ascanio, who was optioned to Iowa.
Samardzija, who last pitched for the big league club on May 5, made 12 starts for Iowa but will be solely a reliever for the Cubs.
Manager Lou Piniella acknowledged the challenge of a pitcher adapting from a starter to a reliever, but he said there was simply no room in the rotation.
"It's a little bit of a problem," Piniella said. "But, look, to prove the pitches like the breaking ball and so forth, that's why they sent Jeff down originally."
While pitching for the Pacific Coast League club, Samardzija went 5-3 with a 3.72 ERA, including one complete game. He also had 53 strikeouts in 67 2/3 innings.
The former Notre Dame receiver struggled with the Cubs in five appearances this season, posting an 8.10 ERA. Last year, Samardzija was 1-0 and had a 2.28 ERA.
In other news, there's still no decision as to where Aramis Ramirez will be sent for a few rehab games. Ramirez, who has been out since May 8 with a shoulder injury, should still be moved on Thursday.
Piniella anticipates Ramirez may only need 15 to 20 at-bats because he needs to see a ball coming at him at full speed. The manager hopes to see Ramirez back for the Braves series, which starts Monday.
"He's done about everything that you can do here," Piniella said. "We talked about a couple months, and it's around that time frame. He's done all his exercises, he's been taking batting practice, strengthening things. The only thing left now is at-bats down at the Minor League level, he's been out so long. That's the final stage."
Wayne Staats is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.