Iwakuma (2-0) got the better of his countryman and Texas starter Yu Darvish in the seventh all-time pitching matchup between the pair, both in Japan and the majors. Darvish (2-1) holds a 4-3 advantage when the pair match up on the mound.
Iwakuma held the Rangers bats silent until the fourth inning when Ian Kinsler led off with a homer. That proved to be the extent of the Rangers' offense as Iwakuma and three relievers combined to shut down Texas.
Tom Wilhelmsen pitched the ninth for his fourth save.
Of Seattle's starting staff, Iwakuma has been one of the most consistent pitchers early in the season. He was perfect through three innings until Kinsler led off the fourth with a towering homer that barely cleared the glove of Raul Ibanez leaping at the wall in left.
It was the most obvious example to date of the new dimensions at Safeco Field being a factor.
A year ago, Kinsler's homer would have either been caught on the warning track or caromed off the wall, which has been moved in slightly and lowered to 8 feet.
Instead of getting rattled, Iwakuma kept rolling. He retired the next five straight and eight of the final 11. Iwakuma left with two outs in the seventh after throwing 90 pitches, giving up just three hits and striking out six.
Texas had a chance to get to Iwakuma in the sixth when Kinsler led off with a single and was called safe stealing second base by umpire Gary Darling when replays clearly showed he was out.
With Mariners fans expressing their displeasure after the missed call was shown on the giant new video board, Iwakuma escaped the jam getting Lance Berkman and Adrian Beltre to pop out to the infield to end the inning.
Charlie Furbush took over with two outs in the seventh and allowed a single and a walk, but Stephen Pryor struck out pinch-hitter Craig Gentry to end the inning.
Darvish didn't throw well in Seattle during his first season and didn't start Year 2 much better. Darvish plunked Jason Bay for the first time with one out in the first, then gave up consecutive singles to Kendrys Morales and Ibanez, the second one scoring Bay.
Justin Smoak flew out to deep center field for the second out, but Seager continued to show signs of breaking free from an early slump. Seager lined a shot down the first base line and off the tip of Mitch Moreland's glove for a two-run double.
But that was it for Seattle's offense. Darvish didn't allow a hit after Seager's double and retired his final 12 batters before leaving after the sixth.
Notes: Iwakuma walked Moreland with two outs in the fifth inning, the first walk he's issued this season. His streak of 18 innings without a walk was the second-longest in franchise history. ... In three career starts in Seattle, Darvish has a 7.71 ERA. ... Seattle OF Michael Morse had reduced swelling in his fractured right little finger on Friday and the team is staying with the same timeline that he could be back from the injury in less than a week.