The award recognizes the most outstanding relief pitcher during each month of the regular season as voted on by a panel from Major League Baseball.
The Royals' 25-year-old right-hander pitched in 12 games in September and converted each of his 10 save opportunities.
Soria tossed 13 2/3 scoreless innings, and permitted 12 hits and five walks with 21 strikeouts; he was one of five pitchers with at least 10 innings pitched and no earned runs allowed in September.
Highlights from Soria's month included his outing on Sept. 1 at Oakland when he collected a season-high four strikeouts in two innings to preserve a 4-3 victory for the Royals. In addition, he recorded saves on three consecutive days on two occasions, Sept. 7-9 and Sept. 20-22.
Soria nailed down 20 of his final 21 save opportunities in 2009 to finish the season with 30 saves, becoming the first Royals closer to post back-to-back seasons with 30 or more saves since Jeff Montgomery turned the trick from 1991-93.
Originally signed by the Los Angeles Dodgers as a non-drafted free agent in 2001, Soria finished the season third in the Majors among relievers (minimum 50 innings pitched) with 11.72 strikeouts per nine innings, and seventh in the American League in saves. For the season, Soria went 3-2 with a 2.21 ERA, permitting 44 hits and 16 walks with 69 strikeouts over 53 innings pitched.
Other relievers receiving votes included the Twins' Joe Nathan, the Yankees' Mariano Rivera, the Dodgers' Jonathan Broxton and Francisco Cordero of the Reds.