Mike Zunino was 0-for-17 before getting two hits and driving in two runs. The Mariners have won 11 of their last 14.
Iwakuma (12-6) was masterful while allowing four hits and no walks. Since July 1, the right-hander is 7-2 with a 1.63 ERA, 65 strikeouts and four walks in 72 innings over 10 starts.
The Phillies, who have scored three runs or less 65 times this season, got just one batter to second base against Iwakuma. They have lost six of their last eight.
The Mariners need three pitchers in the ninth to preserve the victory. The last of the trio, Fernando Rodney, came on with two outs and the tying run at the plate and struck out Cody Asche to record his 36th save in 39 opportunities.
The Phillies avoided their 14th shutout when Chase Utley scored on Yoervis Medina's wild pitch. Domonic Brown followed with an RBI double to pull Philadelphia within 5-2 before Medina hit Carlos Ruiz to put runners on first and second for Asche.
A.J. Burnett (6-14) lost his fifth straight decision and dropped to 0-6 with a 6.41 ERA in seven starts since the All-Star break. He allowed six hits with four walks and hit two batters in 7 2-3 innings.
Control has been a problem all season for Burnett, who began the day leading the majors with 72 walks. He started poorly, with eight straight balls to open the game.
The Mariners took advantage as both of those batters, Austin Jackson and Dustin Ackley, scored on RBIs by Robinson Cano and Logan Morrison.
Seager began the sixth with a homer, his 19th, to deep right on a 3-0 pitch. Zunino hit a two-run single in the eighth to chase Burnett and put Seattle in front 5-0.
Mariners: RF Michael Saunders, who has been on the DL since July 11 with a strained left oblique, made his 10th rehab start Tuesday night for Triple-A Tacoma. Entering Tuesday, he was batting .258 at Tacoma.
Phillies: OF John Mayberry Jr. played in his first rehab game with Triple-A Lehigh Valley at Pawtucket on Tuesday night. Mayberry has been on the DL since July 21 with left wrist inflammation.
Mariners: LHP James Paxton (3-0, 2.20) makes his sixth start of the season when the teams wrap up their three-game series on Wednesday afternoon. The 25-year-old is unbeaten in nine career starts, with his latest victory last Friday at Detroit when he allowed one run on five hits in six innings. Paxton will be facing the Phillies for the first time.
Phillies: LHP Cole Hamels (6-6, 2.44) will look for some better luck. He has a 1.71 ERA in his last three starts but is 0-1 over that stretch due to a lack of run support that has plagued him all season.