Gambia midfielder Sanna Nyassi scored goals in the 38th and 66th minutes of the match after the Sounders had fallen behind when Kevin Burns netted in the 24th minute.
Sounders coach Sigi Schmid was very proud of his players after the triumph and also praised the fans, who turned out in full force (31,311).
"I'm extremely proud of our team. We talked about being able to win tonight and doing something special. We're the first MLS team to repeat and the first to repeat in a long time. The guys really took it to heart and even though we went down 1-0, they should a lot of character," Schmid said.
"These fans are the best; they're fantastic. They've stayed here until the end and stayed through the trophy presentation. They'll be able to tell their kids and grandkids one day that they were there the day the Sounders won the second."
And Nyassi was delighted to have given his contribution to the team after finding some good form of late.
"Yeah, it's a good feeling tonight. After breaking the deadlock the other day the chances came tonight and I made sure I finished both of them. Victory for us. We set a record in history, the first [MLS] team to win the trophy twice in a row. Good feeling," he said.
"My confidence keeps growing after scoring two goals tonight. I make sure that I continue working hard at training. Make sure that I do a lot of finishing at training. Whenever the chances come I make sure that I take them."