Porcello (13-8) scattered five hits and two walks, with his only real blemish coming on Abraham Almonte's solo home run.
Torii Hunter and Omar Infante each delivered two hits and an RBI for Detroit, which holds a comfortable lead over second-place Cleveland for the lead in the AL Central. Victor Martinez also knocked in a run in the victory.
Joe Saunders (11-15) was pegged with the loss, allowing three runs on seven hits over 5 2/3 frames. Seattle has dropped seven of its last eight contests.
Saunders retired the first two batters in the sixth, but was unable to finish the inning. He walked Miguel Cabrera, then gave up three straight singles, with Martinez and Infante each driving in runs to give Detroit a 3-1 lead.
Tom Wilhelmsen was able to come in and strand runners at first and second, but the Tigers reached the reliever for a run in the seventh, with Hunter slapping a two-out single up the middle to score Alex Avila and create a 4-1 cushion.
Justin Smoak followed a Raul Ibanez double with an RBI single in the eighth to reduce the margin to two, but Joaquin Benoit closed the door in the ninth for Detroit, securing his 21st save of the season.
In the first inning, back-to-back singles by Hunter and Cabrera put runners at the corners, and Saunders' wild pitch while facing Prince Fielder allowed Hunter to score for a 1-0 Tigers lead.
Almonte's blast deep into the right field seats tied the score at 1-1 in the third.
The Tigers have won five of their last six games ... Prior to Monday's contest, Detroit had dropped nine of its last 12 games with opponents in the AL West ... Almonte, Ibanez and Smoak each had two hits for Seattle.
09/16 22:12:50 ET