Iwakuma (13-6) scattered four hits and two walks while striking out six over eight scoreless innings. He lowered his ERA to 2.76, good for third in the AL.
"He controlled the game," Mariners manager Eric Wedge said of his starter. "Just a great job."
Justin Smoak homered, drove in two runs and scored three times, Franklin Gutierrez hit a solo homer to highlight a three-hit night, and Michael Saunders went 2-for-4 with two RBI for the Mariners, who had lost eight of nine coming in.
Detroit, despite having its three-game win streak snapped, still holds a healthy six-game lead in the AL Central race, as the second-place Indians lost to the Royals.
Verlander (13-12) was touched for three runs on four hits and three walks over seven innings. The former Cy Young Award winner has won just once in his last eight outings.
"I can take a little bit of solace being able to go seven," Verlander said.
Half of the hits allowed by Verlander came in the second inning. Saunders found the left-field gap with a double, and Smoak, who led off the inning with a walk, labored around the bases to score from first.
Saunders scored on Nick Franklin's base hit to left, and Smoak made it 3-0 in the sixth when he sent Verlander's 91 mph fastball the other way and over the left-field wall.
Seattle blew the game open with a four-run eighth. Smoak singled in Gutierrez, Saunders doubled in a run and a wild pitch and an error each plated a run.
Iwakuma worked out of two bases-loaded jams to keep Detroit off the board. In the first, the Japanese right-hander struck out Matt Tuiasosopo to leave the bags full, and in the fourth he escaped unscathed by getting Hernan Perez to bounce into a 6-4-3 double play.
The twin killing was the first of 11 straight batters retired by Iwakuma, who exited having thrown 72 of his 105 pitches for strikes.
Danny Farquhar completed the four-hit shutout in the ninth.
Iwakuma has posted a 0.96 ERA in four September starts. He has not allowed a run in 25 straight innings on the road -- tied for the longest streak in Mariners history ... Verlander eclipsed the 200-inning mark for the seventh straight season ... The Tigers have been shut out three times this month ... Prince Fielder's double in the first inning was Detroit's only extra-base hit.
09/19 00:11:05 ET