Showalter's first game will be Tuesday night at Camden Yards against the Los Angeles Angels.
Baltimore had the worst record in the majors at 31-70 going into last night against Kansas City and is headed toward its 13th straight losing season. The Orioles fired manager Dave Trembley June 4 and replaced him on an interim basis with Juan Samuel.
"Buck Showalter's proven track record makes him the right choice for manager of the Orioles," president of baseball operations Andy MacPhail said in a statement. "We believe Buck's extensive experience and expertise will be a major benefit to us as we look towards a more successful future."
Samuel will return to his job as the team's third-base coach. Baltimore went 16-31 with him in charge. "It's been a good experience," Samuel said. "These guys played for me hard. I know the record doesn't reflect that, but I thank the guys for that."
The rest of the Orioles' coaching staff will stay intact. Gary Allenson, the interim third-base coach, will resume his spot as manager at Triple-A Norfolk.
Showalter's move was first reported by ESPN, where he worked as an analyst. His last television appearance was on Monday.
"My job with ESPN allowed me to follow this organization closely over the last several years, and although the current record may seem to indicate otherwise, I see enormous potential with this club," Showalter said in a statement released by the team. " . . . Baltimore is a tremendous baseball town with passion and pride in its club, and my family and I look forward to making it our new home."
The 54-year-old Showalter also was the first manager for the Arizona Diamondbacks. He was hired well in advance and given time to put the team in position to contend once they started playing in 1998. He was fired after the 2000 season, and Arizona won the World Series the next year.