Dempsey scored the winning goal in the 85th minute when he fired a shot from inside of 6 yards into the center of the net. He had three goals in his last game in Seattle's 4-4 tie with Portland.
''I'm feeling good, I'm feeling I'm in a good rhythm heading into camp on May 15th,'' Dempsey said about his play heading into the World Cup for the U.S. team.
Seattle (3-2-1) handed Dallas (4-1-1) its first loss of the season.
''A lot of people said he was out of form, I guess he's put those people to rest,'' Sounders coach Sigi Schmid said.
Seattle tied it 2 on an own goal by Dallas' Stephen Keel in the 75th minute. The goal came after Dempsey attempted a cross from the right side that bounced off of the side of Keel's leg and into the left corner of the net. Dempsey's first goal came off a free kick in the 22nd minute after he fired a bending shot that landed just inside the goal line to tie it at 1.
''I'm always trying to take the free kick, I'm a little bit selfish,'' Dempsey said. ''I was lucky that the linesman saw it go behind the line, a lot of time they don't see that.''
Michel scored on a penalty kick in the 42nd minute to give Dallas a 2-1 lead. Dallas was awarded the penalty kick after Seattle's DeAndre Yedlin committed a hard foul on David Texeira in the penalty area. Michel converted on the attempt after he faked to his right and fired a shot into the left corner of the net.
The goal was one of 11 shots FC Dallas had in the first half compared to only two for Seattle, for the game FC Dallas finished with a 16-13 advantage in total shots.
''It was a bad night for us, we didn't do what we were supposed to do and didn't capitalize on our chances in the first half,'' Dallas midfielder Je-Vaughn Watson said.
Texeira scored on a header in the 11th minute for the game's first score. Texeira took control off a ball that bounced over the head of Seattle goalkeeper Stefan Frei before knocking in a header from inside of 10 yards and into the center of the net.