The home side flew out of the traps and were back in the tie thanks to goals from Eddie Johnson and Zach Scott. The match took a decisive turn 20 minutes from time when Galaxy were awarded a contentious penalty following a handball by Adam Johansson.
Robbie Keane made no mistake from the spot to ensure Bruce Arena's side will take on Houston Dynamo in a repeat of last year's MLS Cup Final.
Read how a dramatic evening unfolded in Seattle with Sports Mole's live text commentary.
1.58amGood morning MLS fans. It's time to dig deep and prepare for the final Western Conference game of the season. It's heavy underdog Seattle aiming for their Rocky moment against reigning champions LA Galaxy.
1.59amOh and did I mention? LA have a three-goal head start after an utterly dominant display in the first leg last week. Pressure on Seattle!
2.00amSeattle have a genuinely fantastic home crowd and boy are they going to need their full backing tonight. This promises to be an intriguing clash!
2.01amLA have a striker who has peaked just at the right time, it's the Premier League's old flame Robbie Keane. He scored twice last week in the first leg and is primed to put the tie beyond doubt if given a chance tonight.
2.07amEddie Johnson and Fredy Montero both fit and start for Seattle. They need to be on fire and quickly if the comeback is to become a reality.
2.08amThe teams are making their way onto the field, on a bizarre red carpet. This is MLS after all, that quirky touch evident as always!
2.09amOne thing that cannot be poked fun at tonight is a truly fantastic home support in Seattle. The Sounders had sold 43,000 tickets yesterday despite trailing 3-0 on aggregate. Wonderful stuff for a relatively new franchise.
2.10amSeattle fans have quickly become used to feeling like the bridesmaids of MLS. They reached this stage last year and, you guessed it, lost the first leg 3-0.
2.11amWe are moments away from kickoff and the conditions are far from ideal. Torrential rain is in full flow, that should level things up a bit.
1 minWe are underway in the wind and rain of Seattle. Pressure on!
2 minGalaxy are the first side to get the ball down and attempt a forward pass, but Wilhelmsson fails to latch onto to a long pass with a dart inside his full-back.
3 minSeattle get the ball to Montero and he looks dangerous straight away. An inviting cross fails to make contact with Eddie Johnson but the intent is there for all to see.
CHANCEJohnson is played clean through on goal in behind the LA defence. He fails to stay composed and spoons the ball high and wide from close range. Huge opportunity in the early stages.
4 minJohnson is quickly involved again, winning a free kick in an advanced position after a foul from Tommy Meyer.
5 minSeattle are playing a high line and putting pressure on Beckham in midfield from the outset. High risk and very necessary under the circumstances.
6 minRobbie Keane is caught offside twice in as many minutes.
8 minSteve Zakauni uses his pace to win Seattle a corner, but Buddle clears with a bullet header.
10 minSeattle put pressure on high and think they have the opening goal but Eddie Johnson is ruled offside. Replays suggest the linesman got that one wrong. Tough call on Seattle who have made a frenzied start. Montero looks very lively!
12 minWe have a game on our hands here in Seattle. A simple ball over the top splits the LA defence in half and Johnson uses his pace to great effect. A brilliant first-time finish and Seattle are alive and kicking in this tie!
13 minLA put some instant pressure back on and win a corner that is quickly cleared.
14 minMontero is causing a nuisance at every opportunity. This time he makes contact with a header but there isn't enough power behind it to trouble Josh Saunders.
17 minSeattle are dominant in possession and LA look a nervous wreck at the back. A recipe for disaster. Becks and some of the more experienced players need to calm this down for the reigning champs.
18 minSpeaking of the former England skipper, he has barely had a kick tonight thanks to that high-pressure style Seattle have been forced to employ.
19 minSeattle are targeting Meyer in defence and it's working well at this stage. He loses possession and eventually LA concede another free kick in their defensive third.
21 minJust a reminder that Seattle need two more goals to restore parity in this tie. Plenty left to play for and the atmosphere at CenturyLink Field sounds fantastic.
22 minSaunders comes out to collect after some more promising work from Montero and Tiffert.
23 minFulham and Cardiff City fans will hardly believe their eyes if they turn this match on. Eddie Johnson showing pace, power, poise and presence. And I'm only half joking with that alliteration attack.
26 minA last-ditch tackle from Gonzalez stops Johnson from getting a shot on target from close range.
27 minBeckham finally gets some possession in a useful position, but after a neat exchange with Magee the Seattle pressure cooker comes into play once again.
28 minA dangerous backpass is only partially cleared and Beckham goes for shades of Selhurst Park from range but can't quite make the desired connection.
29 minBuddle meets a cross but has his back to goal and tries to turn the ball towards Magee. His pass is short and the half-chance is gone.
30 minZakuani tries a nice reverse pass into Evans but his shot is deflected wide for a corner.
31 minA superb Alonso effort from range needs a strong hand from Saunders to turn it round the post.
33 minMontero tries a flick but gets it all wrong, probably for the first time tonight.
35 minLA can't maintain any form of consistent time on the ball but Keane uses his experience to win a cheap free kick and buy his defence a breather.
36 minAlonso goes for goal again 30-yards out but fails to trouble Saunders with a wayward drive.
37 minJohansson wins Settle a fourth corner of the night but again the quality of delivery is not up to standard.
38 minMarcelo Sarvas tries a lofted ball over the top but Keane is caught unaware and the chance is quickly gone.
39 minWonderful invention from Keane creates LA's best chance of the next. He sells the full back a dummy before crossing for Buddle who is inches away from turning home unmarked. Warning signs for Seattle, who need to take this lead into the break.
40 minLA are getting more time on the ball, maybe signs the Green machine is slowing down as we edge towards the break.
42 minHuge penalty appeal from LA as Magee and Keane hound the referee after a handball claim in the area, but their protests are waved away.
43 minCloser inspection suggests Hurtado had his hands by his side and could do little about the contact.
45 minBeckham gets free and plays in Buddle out by the corner flag but the referee blows for the end of the first half.
Half TimeSigi Schmid can be happy his side are still alive thanks to Eddie Johnson's early strike. This tie is still alive and kicking.
3.03 amA lot to admire about Seattle's desire to keep their season alive in the opening 30 minutes. A more clinical finisher next to Johnson and Montero and they could well be in business here.
3.04amThe longer this match goes you have to feel LA's experienced heads will produce a moment of quality.
3.05amJohnson is very unlucky not to have a brace to his name at the interval after being wrongly ruled offside just minutes in.
3.07amA reminder that LA talisman Landon Donovan has not even made the bench tonight as he continues to nurse a hamstring strain.
3.08amSpeaking of Galaxy fitness worries, David Beckham left the pitch limping heavily at the break. Interesting to see if he is capable of playing 90 minutes tonight.
3.11amWhile we're killing time looking at the bigger picture, tonight's winner will face Houston Dynamo in the MLS Cup Final. They secured their place in the showpiece earlier today and could meet LA in a rematch of last year's final.
3.12amOne player who has struggled tonight for LA is Edson Buddle, cutting an isolated figure in attack and failing to hold the ball up for any prolonged period. Bruce Arena needs a word at the break with his powerful striker.
3.15amTime to get the second half underway, Seattle need two goals to keep their season alive. Anything less and LA will advance to defend their title in a rematch with Dynamo.
45 minBeckham is back out on the field and seems fit to continue, for now.
46 minEddie Johnson is penalised for a foul in the area after a creative burst from Montero.
47 minSaunders punches a cross clear but Seattle pump the ball back into the danger zone. The referee saves the keeper's blushes by awarding LA a soft foul.
48 minMontero tries an early cross but Meyer does well to head clear at an angle.
50 minZakuani drives down the wing but is shocked not to be awarded a corner after trying to play the ball off Franklin.
52 minJohnson is in behind Meyer again and bears down on goal, but the defender does well to recover and avert danger with a strong block.
53 minJohnson tries to play Montero in behind and Omar Gonzalez dithers in defence before eventually hammering the ball out for a throw.
55 minGonzalez and Meyer have look uncomfortable all night as a central-defensive pairing. Admittedly they have been little protection but their lack of pace as a unit has been evident all game.
56 minBuddle wins LA a corner, forcing Scott into a mistake.
57 minBeckham takes it short and doesn't meet the return. Poor decision there from LA.
58 minSeattle break and win a corner of their own in response. Tiffert finds Zach Scott who scores. GAME ON!
59 minScott uses a box full of players to lose Edson Buddle and guide home at the neat post. Intelligent play rewarded for the full back and Seattle are one goal away from completing a remarkable turnaround.
60 minSeattle are wasting no time and pump the ball back into danger from their next possession. LA look rattled and need to change their approach or risk imploding.
61 minMontero robs Sarvas and Seattle are back on the attack.
LA Sub:Juninho is coming on for the ineffective Wilhemlson. He will come into central midfield to try to help Galaxy keep the ball and stop the Seattle tide.
62 minLA have a free kick about 35 yards from goal. Beckham lines it up and plays the ball into a crowded area but Seattle clear without pressure.
64 minSeattle win a free kick in a potent position. Alonso is fouled by Juninho on the edge of the penalty area. Montero fancies this.
Free KickMontero's shot hits the wall and fails to make an impact.
PENALTYLA break and Robbie Keane plays the ball off Johansson to win a controversial penalty. It's an absolute shocker of a decision.
GOALRobbie Keane steps up and slots home with minimal fuss. Awful luck on Seattle but a player of Keane's calibre is not going to pass up a chance like that. 1-1 on the night and the Galaxy are firmly back in control.
69 minSeattle's momentum has been completely taken away with one swift decision from the official. Tough for the Sounders to pick themselves up and find two goals in the remaining 20 minutes.
70 minThe home side are yet to make a change, time to freshen things up and go for broke.
71 minMontero falls to the ground in desperation to try to win a penalty. Clear dive in Beckham's opinion and he says as much to the referee.
Seattle Double SubMontero makes way for Mario Martinez while Estrada comes on for the guilty party Johannson.
74 minAlonso receives a yellow card he has to take. A cynical foul on Juninho to prevent the breakway after the Brazilian robs possession brilliantly in midfield.
75 minEstrada tries to link up with Johnson, who is slow to react. The opening goalscorer has been a peripheral figure in this second half.
77 minKeane brings down Martinez and Seattle have a free-kick. Tiffert's delivery is dropped by Saunders but LA are let off the hook as Meyer is first to react.
78 minEstrada takes a quick free kick but Zakuani tries to go it alone and is quickly dispossessed.
79 minMagee breaks for LA, cutting inside and forcing Gspurning into a smart stop from close range.
80 minScott fires in a lofted cross but Beckham clears and brings the ball away.
81 minEstrada keeps in a long pass and finds Zakuani, but his cross falls kindly for Saunders.
Seattle SubLast throw of the dice for the home side as an exhausted Zakuani makes way for Mauro Rosales.
83 minMartinez tries to bring the ball forward for Seattle but they have no target man to hit.
84 minRosales has a horrible first contribution, launching a wayward cross out for a goal kick.
LA SubEdson Buddle is withdrawn for Michael Stephens. Shut up shop time for the Galaxy.
86 minKeane concedes another foul and is lucky not to pick up a yellow card.
87 minSeattle pump the ball long and win a free kick just outside the area. Silly foul from Sarvas on Martinez to give away the dangerous set piece. It's now or never for Seattle.
88 minTiffert strikes a tame ball straight into Saunders' grateful arms and the chance is gone.
89 minRosales finds Jeff Park in space deep into the penalty area but his powerful header drifts wide.
90 minEstrada has time and space in the area but he can only blast a shot down the centre for Saunders to parry away.
91 minFour minutes of added time. Rosales measures another inviting cross but LA clear all to easily.
92 minSaunders collects another long ball and the job is nearly done for the visitors.
93 minLA win a throw in the opposition corner in the final minute. Seattle are all but done.
4.07amSounders skipper Alonso has been shown a red card for his protests. Seattle are still livid about the penalty decision that went LA's way.
4.08amLA rode their luck and a wave of Seattle pressure to retain their western conference title. They will now entertain old rivals Houston Dynamo in the MLS Cup Final in two weeks time.
4.09amA rain-soaked Seattle is about to have the trophy presentation 40,000 home fans did not want to see.
4.10amBeckham, Keane and the Galaxy are milking this for all it's worth. In truth the damage was done in last week's first leg.
4.11amKeane picks up the trophy for under a second before darting off, with the rest of the Galaxy squad quick to follow. A real sense of job still to do for LA against a tough Dynamo side in a repeat of last year's final.
4.15amLady luck is really shining on LA, who will host the one-off final at the Home Depot Centre in two weeks time.
4.16amThat's all for tonight football fans, thanks for joining us here at Sports Mole. Sleep well, or alternatively insomniacs, switch over to our ball-by-ball coverage of the cricket. The choice is yours!