Mike Yeo was always next in line, but he's taking over sooner than originally planned.
When Yeo was hired as associate coach last summer, Armstrong initially said the former Minnesota Wild head coach would succeed Hitchcock as Blues bench boss after this season and "for three years after that."
However, the move was made now with St. Louis having lost five of six games and sitting last in the NHL with a team save percentage of .887, according to Corsica Hockey.
Hitchcock led the Blues to the playoffs in each of his five previous seasons behind the bench, but their run to the Western Conference Final last spring was the furthest he'd take them.
The 65-year-old ranks fourth on the NHL's all-time wins list with 781 - only a single victory behind Al Arbour - in 1,453 regular-season games.
Prior to his Blues tenure, Hitchcock spent parts of four seasons with the Columbus Blue Jackets, parts of four campaigns with the Philadelphia Flyers, and seven seasons with the Dallas Stars, leading them to a Stanley Cup championship in 1999.
He was named Blues head coach following the firing of Davis Payne early in the 2011-12 campaign.