Elliott, 26, is enjoying his best season, being named to the All-Star Game while racking up a 15-5-1 record in 22 games for the surging Blues.
He has compiled a 1.68 goals-against average while allowing the fewest goals (36) and absorbing the fewest losses among regular NHL goaltenders.
"Brian came in here and helped stabilize our goaltending and make it one of the top tandems in the NHL," Armstrong said. "Having Brian and (Jaroslav Halak) signed through the '13-'14 season, our goaltending is set."
Elliott has also played with the Ottawa Senators and Colorado Avalanche during his four-year career, posting a 76-58-17 record In 164 career games (151 starts).