They will fight for the title during UFC 129 Saturday night at Rogers Centre.
St. Pierre weighed in at 169½ pounds. Several seconds earlier, Shields did his part to make the fight official by checking in at 169.
Despite being the object of a hostile Canadian crowd, Shields expressed confidence that he will realize his longtime goal of becoming UFC 170-pound champion.
"I am bringing the belt home," said Shields, who is a native of Mountain Ranch, Calif. "The crowd can boo me all it wants. It will only be me and him in the cage."
St. Pierre wasn't impressed with Shields' comment. When his turn came to address the crowd of more than 4,000, the Montreal resident assured his supporters there was little need to worry.
"It's going to go down with a victory," St. Pierre told his adoring fans.
There was some suspense at the weigh-ins.
Former WEC lightweight champ Benson Henderson weighed a half-pound over the 156-pound requirement.
In was clear before Henderson stepped on the scale that weight might be an issue for him.
Henderson immediately requested that a towel be used to block onlookers as he took off all his clothing. Normally, a fighter will only remove all clothing if he fails to make the required weight on his first attempt.
Though Henderson was unable to make weight during the official weigh-in, he will have a few extra hours to comply. If Henderson is still can't make weight, he will likely receive a fine but be allowed to compete Saturday night.
His opponent, Mark Bocek weighed in at 155 pounds.
Every other fighter on the main card made weight.
Couture says this will be his final professional MMA fight. Machida is looking to end a two-fight losing skid.
Franklin McNeil covers mixed martial arts and boxing for ESPN.com. Follow him on Twitter at www.twitter.com/Franklin_McNeil.