This is Showalter's third Manager of the Year award, having previously won in 1994 and 2004. He is the sixth manager with three or more Manager of the Year awards.
Showalter received 25 of 30 first-place votes and 132 points in balloting by the Baseball Writers' Association of America. He kept up his pattern of winning the award once a decade following victories with the New York Yankees in 1994 and Texas in 2004.
"I won't be doing it 10 years from now,'' Showalter said on the MLB Network telecast.
The Los Angeles Angels' Mike Scioscia was second with four firsts and 61 points, and Kansas City's Ned Yost third with 41 points. Seattle's Lloyd McClendon followed with 29 points.
Showalter guided the Orioles to a 96-66 record and their first AL East title since 1997. Voting took place before the playoffs, where Baltimore swept Detroit in the Division Series and then was swept by Kansas City in the AL Championship Series.
Until the ALCS, the Orioles had not lost four in a row since May and had not dropped consecutive home games since June 28-29.
Schowalter and the Orioles had to overcome key injuries to catcher Matt Wieters and third baseman Manny Machado this season. First baseman Chris Davis was suspended 25 games that carried into the postseason for a failed drug test.
Showalter became the third Orioles winner, following Frank Robinson in 1989 and Davey Johnson in 1997.
"It's such a great reflection on our organization,'' Showalter said. "It's pretty humbling."
The Associated Press contributed to this report.
Buck Showalter Named AL Manager Of The Year
Jerry Crasnick discusses Baltimore Orioles manager Buck Showalter winning the American League Manager of the Year award.