The A's sent outfield prospect Dairon Blanco and right-handed pitching prospect Ismael Aquino to Kansas City.
Diekman, 32, is 0-6 with a 4.75 ERA and 1.34 WHIP in 48 appearances. He's had a string of strong performances of late, giving up only one run in his last six appearances, and has struck out 63 batters in 41.2 innings.
Kansas City traded Homer Bailey to Oakland earlier this month, as the two sides have become strong trade partners. Ken Rosenthal of The Athletic reported the A's are expected to remain active this week looking for a starting pitcher. The New York Mets' Zack Wheeler and Cincinnati Reds' Tanner Roark are reportedly among their targets.
Oakland is second in the AL West with a 58-47 record but has lost five of its last six games. The A's are half a game behind the Tampa Bay Rays for the second wild-card spot in the AL and should battle with Tampa and the Boston Red Sox down the stretch for the berth.
Adding another starting pitcher would help the cause, as the A's rank just 13th in starter ERA. Diekman will give them one of the better bullpens in baseball and a necessary left-handed power arm. He joins a hard-throwing quartet in the back half that includes Blake Treinen, Liam Hendriks and Lou Trivino.
The A's and Diekman have a mutual option for 2020 on his contract.