Beckham batted .179 with one homer and four RBIs in 23 games before going on the DL with a groin strain in April. After undergoing core surgery, he missed two months.
Beckham said Monday that he had been playing with pain before he came from Tampa Bay to Baltimore last July, noting, "It's something I battled through for years."
With Beckham back, the Orioles started all their front-line players for the first time. The lineup against Seattle on Monday night included Jonathan Schoop, Mark Trumbo and Colby Rasmus, all of whom have spent extensive time on the disabled list.
Chris Davis, who sat out eight games in an effort to shake a lengthy slump, said, "I'm glad to see everybody back."
A full complement of players failed to give Baltimore (23-54) a win, as the team fell 5-3 in the series opener against the Mariners and Beckham went 0-for-4.