Chris Young (2-0) allowed two hits over six strong innings to win his second straight start. He began the year with four straight no-decisions as he came back from shoulder surgery that caused him to miss much of 2013.
The right-hander faced the minimum through three and didn't allow a hit until Jed Lowrie's one-out single in the fourth. Brandon Moss hit a tying two-run homer two batters later.
Romero, a 25-year-old rookie, homered in his 53rd career at-bat. He is batting .306 over his last nine games.
Moss hit his drive to right to give the A's brief momentum only to see Romero connect in the top of the fifth for a Seattle lead. Brad Miller added an RBI single in the sixth.
Scott Kazmir (4-1), off to the best start in his career, was tagged for eight hits and four runs in six innings.
The A's were without two key outfielders and hitters to begin the game with the injured Yoenis Cespedes and Josh Reddick out of the starting lineup. Cespedes entered as a pinch-hitter in the seventh and stayed in the game.
Oakland did little else after Moss' big hit and kicked off a 10-game homestand the club's first of three such stints at the Coliseum this season with just its third loss in seven matchups against Seattle so far.
Young won back-to-back starts for the first time since May 19 and 24, 2009, for the Padres. He also improved to 14-6 in 29 career May starts. His April 6 outing at Oakland was his first in the majors since Sept. 29, 2012, with the New York Mets.
Fernando Rodney, Seattle's fifth reliever on a night the bullpen was thin, finished for his eighth save in nine opportunities as Seattle earned its first three-game winning streak since sweeping a season-opening series at the Angels.
NOTES: Mariners manager Lloyd McClendon expected to name his second starter for Wednesday's doubleheader on Tuesday. Felix Hernandez will start one of the games. The teams must make up a game from April 4, when Oakland didn't use a tarp through a heavy rainstorm and the field was unplayable. ... Oakland RHP Sonny Gray (4-1, 1.91 ERA) earned AL Pitcher of the Month honors for April. "Every time I take the mound, it's fun," Gray said. ... A's RF Reddick is unlikely to return to the lineup for a couple of days as he nurses a sprained ankle suffered Sunday in Boston. ... LF Cespedes wanted to play but manager Bob Melvin opted to give the slugger another day to rest a tender hamstring, though he was available off the bench. ... Mariners OF Logan Morrison, sidelined since April 16 with a strained right hamstring, is recovering more slowly than expected. Even McClendon is surprised. "I have been. There's no sense in arguing about it," he said. "When I get him I get him. Until then I'll just read his tweets." ... Seattle recalled OF James Jones from Triple-A Tacoma and optioned OF Abraham Almonte to Tacoma. ... Oakland last had three or more 10-game homestands in 2002, with four that season.