Baltimore has won nine of its past 11 games and remained atop the AL League East. An RBI single by Manny Machado in the third, snapped the Orioles' scoreless streak at 19 innings,
Chen (12-3) allowed one run and five hits with one walk over 7 1-3 innings. Andrew Miller, acquired from Boston on Thursday, pitched a scoreless eighth in his debut for the Orioles (61-47). Left-hander Zach Britton picked up his 22nd save.
Robinson Cano had two hits for the Mariners and is 115 for 317 (.363) lifetime at Camden Yards.
Mariners left-hander Roenis Elias (8-9) allowed two runs and seven hits with seven strikeouts and one walk over 5 2-3 innings. It was his longest outing since June 28 against Cleveland.
The Orioles took a 1-0 lead on Machado's single in the third. The Mariners answered the next inning on a double by Kendrys Morales that scored Cano when Orioles catcher Caleb Joseph could not hold onto a relay throw by J.J. Hardy. That snapped a streak of 11 consecutive scoreless innings by Chen against Seattle.
In the sixth, Machado led off with a double and Hardy drove him in with a two-out single for a 2-1 lead.
Seattle fell to 5-37 when trailing after seven innings.
Orioles: Right-hander Ubaldo Jimenez will make a rehab start for Triple-A Norfolk on Sunday. He is then expected to rejoin the Orioles when they travel to Toronto for a three-game series beginning Tuesday.
Mariners: Left-hander James Paxton (2-0, 2.25 ERA) will make his first start after initially going on the disabled list on April 9 with a left lat strain and then going on the 60-day DL with a left shoulder injury.
Orioles: Right-hander Miguel Gonzalez (5-5, 3.77 ERA) will be facing the Mariners for the second time in his career. The previous start was less than a week ago when he did not get a decision in the Orioles' 3-2 victory.