The Galaxy midfielder beat Seattle goalkeeper Kasey Keller from 25 yards out, firing inside the near post for the goal that secured the win on a rainy night in Seattle.
The victory was a good start for the Galaxy in what is slated as David Beckham's final MLS season.
The former England captain's contract expires at the end of this campaign, and he's hoping he can help the Galaxy to an MLS title that would see him add to the six English Permiership championships he won with Manchester United and a Spanish league title won with Real Madrid.
Sounders fans booed Beckham lustily every time he touched the ball. He spent much of the match calling for crosses from his teammates and received a yellow card for a hard tackle in second-half injury time.
The Galaxy garnered the victory despite the absence of new striker Juan Pablo Angel, defender Omar Gonzalez and starting goalkeeper Donovan Ricketts.
Angel was ill, while Gonzalez is nursing a shoulder injury and Ricketts a sore hamstring.
The Sounders had controlled the run of play for most of the match when Los Angeles pounced on a turnover by Seattle midfielder Erik Friberg to score.
Friberg's pass was intercepted by Todd Dunivant, who carried the ball into open space and laid off a pass for Juninho.
Juninho's curving shot was barely within Keller's reach, but the veteran keeper couldn't push it wide of the post.
The Sounders had opportunities to even the score, the best in the 72nd minute when O'Brian White redirected Steve Zakuani's cross, only to see it bounce wide of the far post.
It was the third straight season that the Sounders' had opened the MLS season, and it was the first time they launched their campaign with a defeat.
Despite the rain, 36,433 vocal, scarf-waving fans turned out, but were let down by the Sounders' inability to finish.
The Galaxy continued their domiance of Seattle with a fifth straight victory. They have out-scored Seattle 11-2 in those games, including a playoff triumph over Seattle last season.
Last May, the Galaxy beat Seattle 4-0 in a regular-season contest, a defeat that prompted Seattle management to offer refunds to season ticket holders who renewed for the 2011 season.
Tuesday night wasn't that embarrassing for Seattle, but it was still a tough start for a club that now visits New York on Saturday before hosting Houston the following Friday.
Even before the game began Seattle were on the defensive as they announced that striker and designated player Blaise Nkufo and the team were parting ways.
Nkufo signed with Seattle in early 2010 and joined the team after playing for Switzerland in the 2010 World Cup.
The Sounders said his departure was a mutual agreement.
Beckham's Galaxy open season with win
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