The Cubs also will receive right-hander Dan Straily and 19-year-old outfielder Billy McKinney, the A's first-round pick in the 2013 First-Year Player Draft.
The deal was first reported by FOX Sports' Ken Rosenthal on Twitter. The Cubs and Athletics confirmed the deal Saturday.
The A's, swept earlier this week by the Tigers, may have decided that their rotation could not match up with Detroit in October. With the best record and run differential in the Major Leagues, A's general manager Billy Beane decided to make a move before the July 31 Trade Deadline.
They landed two of the best pitching options available on the trade market. Samardzija has been one of the National League's top starters this season, compiling a 2.83 ERA in 17 starts. He was scheduled to start Saturday against the Nationals; now, he will likely start Sunday for the A's.
The Cubs did discuss a contract extension with Samardzija -- a former Notre Dame wide receiver -- but the two sides were never close on an agreement. The right-hander will be under the A's control through 2015, and it seems unlikely they would be able to extend his contract, given their historically limited payrolls.
In the past three years, the Cubs have dealt two starting pitchers each July and loaded up on talent.
||SS Addison Russell, RHP Dan Straily, OF Billy McKinney
||RHPs Jeff Samardzija and Jason Hammel (A's)
||RHPs C.J. Edwards, Justin Grimm and Neil Ramirez, and 3B Mike Olt
||RHP Matt Garza (Rangers)
||RHPs Jake Arrieta and Pedro Strop
||RHP Scott Feldman and C Steve Clevenger (Orioles)
||RHP Kyle Hendricks and 3B Christian Villanueva
||RHP Ryan Dempster (Rangers)
||RHPs Jaye Chapman and Arodys Vizcaino
||LHP Paul Maholm and OF Reed Johnson (Braves)
Hammel, signed to a one-year contract as a free agent, has been a revelation this season. He's 8-5 with a 2.98 ERA, and was the winning pitcher on Friday in Washington. The right-hander complained after the game that manager Rick Renteria lifted him after 92 pitches. The Cubs have paid close attention to pitch counts for both Samardzija and Hammel, anticipating the chance that they could deal them.
In 2012, Cubs president of baseball operations Theo Epstein dealt Ryan Dempster and Paul Maholm, and last year the team traded Matt Garza and Scott Feldman. The moves have all been part of Epstein's rebuilding effort.
The addition of Russell is intriguing, since the Cubs signed Starlin Castro, 24, to a seven-year contract in 2012 and have two talented middle infielders at Triple-A Iowa in shortstop Javier Baez and second baseman Arismendy Alcantara.
Russell has battled hamstring injuries this year. He was sidelined in Spring Training and then again after the second game of the season, but recently was promoted to Double-A Midland where he has hit .333 with a .939 OPS and three stolen bases in 13 games.
Along with first baseman Anthony Rizzo, Castro is the only Cubs player signed to a long-term contract, but the young infielders seem to make him expendable.
The emergence of right-hander Jake Arrieta most likely made Epstein more willing to deal Samardzija. The Cubs still face long-term questions about building a supply of pitchers to work in front of the young lineups they will feature in upcoming seasons.
Having lost A.J. Griffin to Tommy John surgery and with left-hander Drew Pomeranz on the shelf, the A's have been going with Sonny Gray, Scott Kazmir, Jesse Chavez, Tommy Milone and Brad Mills in the rotation. They have tried to give Gray extra days whenever possible because he has never thrown more than the 182 innings he threw combined in Triple-A and the big leagues last year. Chavez, a career reliever, has been fantastic in the rotation, but had never topped 130 innings at any level in one year and already is at 103 for the season. He's been hit hard in his last two outings.
With the trade, the A's can now deploy a rotation that can match the Tigers -- or any team -- with Gray (8-3, 3.08 ERA), Samardzija (2-7, 2.83), Kazmir (9-3, 2.61), Hammel (8-5, 2.98) and Milone (6-3, 3.55). Chavez can slide back to the bullpen, which also just received a boost with the return of left-hander Eric O'Flaherty.
Combined with the league's top-scoring offense, the A's suddenly look like the team to beat in the American League.