The Blues had offered Pronger $7.22 million, rather than risk losing him to free agency after this season.
"Chris has been a great player for us," said Blues General Manager Larry Pleau of Pronger's 10 seasons. "He's been a workhorse, logs a lot of ice time, played through injury."
However, Pleau said, Pronger's desire for a long-term contract "was something we couldn't do."
The 6-foot-5 defenseman said every Canadian kid dreams of playing at home, but it will be difficult leaving the Blues.
"I have a lot of great friendships from here, I met my wife here and I grew up from a little boy to a man, really maturing in front of everybody's eyes here. I'll miss it," Pronger said. "I'm an Oiler now."