The Red Bulls will have a chance to win their first MLS Cup title after a 1-0 win at Orlando City.
The Red Bulls have only once, in 29 years, reached the final.
Galaxy will have home field advantage, as the higher seed, after an 85th-minute goal from Serbian forward Dejan Joveljic earned them a 1-0 win over the Seattle Sounders.
A 47th-minute header from Andres Reyes gave the Red Bulls the victory and sent the New Jersey club into the December 7 final.
The Red Bulls qualified for the playoffs with a seventh place in the regular season, earning them the final automatic spot in the playoffs.
Sandro Schwarz's team then pulled off a major upset by beating defending champions Columbus Crew in the best-of-three first round before eliminating local rivals New York City last week.
Orlando had the early pressure and Red Bulls keeper Carlos Coronel had to be at his best to keep out an effort from Facundo Torres after a counter-attack.
But Red Bulls threatened themselves when their Swedish playmaker Emil Forsberg floated a free-kick into the box and Belgian Dante Vanzeir flicked the ball just wide.
Two minutes after the interval, New York grabbed the decisive goal when John Tolkin's free-kick was delivered perfectly to the back post, where Reyes arrived to head in.
Orlando grew increasingly desperate in their attacks -- Duncan McGuire got round the back of the Red Bulls defence but fired his shot well wide and then Luis Muriel's rushed effort from a promising position flew harmlessly off-target.
The last chance of the game fell to Ivan Angulo but he too failed to find the target as the Red Bulls, founding members of MLS in 1996, held firm.
"I'm very proud of the whole group. It's amazing to create this atmosphere and also with this fighting spirit. Amazing the last couple of weeks and now we have one week more and it's not done," said German Schwarz, who was the former Mainz and Hertha Berlin coach.
The Galaxy, record five-time MLS champions, will have a chance to extend that mark when they take part in their first MLS Cup final in ten years.
Jovejlic had twice seen efforts saved by Seattle keeper Stefan Frei but he kept his composure to settle a dour contest.
Alex Roldan gave the ball away for Seattle and LA's Spanish midfielder Riqui Puig played a deft through ball to Joveljic, who provided an ice cool finish.
It had been a tight, tactical contest, with the Galaxy, who had scored 15 goals in their three previous playoff games this season, finding their potent attack restricted by Seattle's well organised defence.
Seattle's best chance had seen forward Jordan Morris lay off to Albert Rusnak but the midfielder, in a great position, fired straight at Galaxy keeper John McCarthy.
The run to the Western Conference title comes after the Galaxy, who enjoyed such dominance in the years with Landon Donovan, David Beckham and Robbie Keane, finished next to bottom of the Western Conference last season.
Coach Greg Vanney, who had faced criticism from fans last season, said his team had been rewarded for their concentration against the Sounders.
"There were stretches where we just had to defend and protect our goal. We knew they were a stingy defense, they aren't the best defense in the league for nothing," Vanney said.
"So we knew it was going to be a challenge, we had to stick with it and we got it... the guys have been doing it all year, I'm super proud of them. It was just a matter of getting through."
