David Texeira opened the scoring for the Texan club in the 10th minute at Toyota Stadium before Dempsey levelled 12 minutes later with a sublime free-kick which cannoned in off the underside of the crossbar.
A penalty from Michel three minutes before the break had Dallas ahead again but Stephen Keel's 75th-minute own-goal drew Seattle level.
Enter Dempsey, who netted a hat-trick last week, to save the day for Seattle by playing a neat one-two with Chad Barrett to finish with ease from inside the six-yard box.
The win was Seattle's third of the season, which condemned Dallas to their first defeat, and Dempsey took great pride in helping the Sounders to a vital victory in his home state.
"It's always good to perform well in Texas, because that's where I developed my game in Nacogdoches and driving to Dallas playing my club ball," Dempsey told reporters after the game.
"A lot of friends and family were here, and it's nice to perform well. But the most important thing is we got the win for the team, and we need to keep getting points like that. I think you'll see us start to put in better performances, and we'll tighten up and we'll get it right."
The USA national team captain added: "I'm feeling good.
"More importantly, we got the win today. It wasn't pretty. I thought Dallas played really well. We've got to work on some things, but I thought we showed a lot of character coming back twice in the game, and ultimately getting the win, so that's a good thing to build upon. We'll sharpen up and get better."
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