In exchange for Guerin, the Blues received a first-round draft pick in 2007, forward Ville Nieminen and prospect Jay Barriball. Guerin, 36, is making US$2 million this season and will become an unrestricted free agent July 1.
He has revitalized his career in his first season with St. Louis, scoring 28 goals and 47 points in his 14th NHL campaign. The bruising power forward waived his no-trade clause to leave the Blues for the Sharks, who expect to contend for a Stanley Cup championship.
Heading into Tuesday's play, the Sharks were tied for fifth in the Western Conference with a 38-24-1 record and 77 points.
"I'm thrilled," Guerin said. "This is one of those teams that was on my radar. I am very happy to be going there. You have certain expectations and places that you want to go, so it was easy (to agree to the trade). When it was presented to me, it was a no-brainer."
The Blues are in sell mode. They're out of the playoff picture with a 26-27-9 record and 61 points.
"A big trade always shakes up a team," Guerin said of the Blues. "They've got very professional guys here, resilient guys, guys that have bounced back from a lot. We dug ourselves out of a huge hole that we created at the beginning of the year.
"There are some character guys, some quality guys here. Maybe it stings right now but nobody here in St. Louis is going to roll over."
Guerin says he'll play wherever he's needed in San Jose.
"They've just got so much talent up front that it doesn't really matter," Guerin said. "Wherever (coach Ron Wilson) wants to put me, wherever he thinks I'll fit best. Because I know they've been having success this year, wherever I go, that'll be just fine - we'll make it work."
In 1,010 career games with New Jersey, Edmonton, Boston and St. Louis, he has collected 356 goals and 354 assists.
Two days earlier, the Sharks acquired veteran defenceman Craig Rivet from the Montreal Canadiens for defenceman Josh Gorges and their own first-round pick. The moves signal a change in philosophy for Sharks general manager Doug Wilson, who gave every opportunity to his rich stable of young talent.
But San Jose's youth has been evident in an inconsistent season despite the club's impressive overall record. The Sharks, who lost 3-2 to Anaheim on Monday night, have lost four of six to slip into a second-place tie with Dallas in the Pacific Division.
With NHL leader Nashville acquiring Peter Forsberg and Detroit adding Todd Bertuzzi, the Sharks apparently felt they had to make a move to keep up.
So Wilson used San Jose's stockpile of draft picks and prospects to add two playoff-tested veterans for the season's final 19 games - even if they're only rentals. Both Guerin and Rivet can be unrestricted free agents after the season, but Guerin has won a Stanley Cup with New Jersey, and Rivet has been a solid contributor to the Canadiens for 11 seasons.
Guerin will be the Sharks' oldest player - more than three years older than Rivet. Guerin also played with Sharks centre Joe Thornton for most of two seasons in Boston.
The Sharks acquired Nieminen last season from the New York Rangers, but the Finnish forward has been in coach Ron Wilson's doghouse all season.
Nieminen has a goal and an assist in 30 games this season. Barriball, a forward who was San Jose's seventh-round pick in 2006, is currently playing with the University of Minnesota.
It's the second big move for the Blues in the last few days. On Sunday, they traded forward Keith Tkachuk to Atlanta.