Beckett (4-3) allowed one hit in six innings – an infield single off his glove by Ichiro Suzuki leading off the game – before giving up three runs in the seventh on homers by Russell Branyan and Casey Kotchman.
Still, it was a sharp turnaround from his recent struggles. In his previous three starts, Beckett was 0-2 with a 10.69 ERA.
For five innings, he was engaged in a duel with David Pauley (2-5) whose only major league wins came in his previous two games. Pauley allowed two hits until getting knocked out of the game in the sixth.
Jonathan Papelbon pitched a perfect ninth for his 32nd save in 38 opportunities.
Jon Lester was scheduled to face Felix Hernandez in the night game, a makeup of Tuesday night's rainout. It rained heavily Wednesday morning but only a light mist fell at times during the opener.
Boston won its fourth straight game and improved to a season-best 19 games over .500 but still trails the New York Yankees and Tampa Bay Rays by 5 1/2 games in the AL East.
Seattle lost its fourth straight.
Pauley pitched a total of nine games for Boston in 2006 and 2008 and he returned to Fenway Park where he baffled hitters from the start. He retired the first nine batters and gave up only a double to Marco Scutaro in the fourth and a single to Bill Hall in the fifth.
Then the Red Sox teed off in the sixth.
Scutaro led off with a single to third and took second on Victor Martinez's one-out single to left. David Ortiz then walked, loading the bases. Adrian Beltre followed with a hard shot to the mound off Pauley's glove that bounced toward the third-base line as Scutaro scored the first run.
Mike Lowell hit a sacrifice fly to right and Daniel Nava made it 4-0 with a two-run lined single over the head of second baseman Chone Figgins. Jamey Wright replaced Pauley and struck out Hall to end the inning.
Beckett kept up his dominance by starting the seventh with his seventh strikeout, fanning Figgins. Then, like Pauley, he quickly lost his touch.
Branyan his 20th homer of the season to right field, Jose Lopez singled off the left-field wall and Kotchman followed with a two-run shot to right field, his ninth. Suddenly, it was 4-3, Beckett was done after allowing four hits and one walk and Daniel Bard was coming in from the bullpen.
Boston's best reliever did the job quickly, fielding grounders by Franklin Gutierrez and Josh Bard and throwing to first to end the inning.
The Red Sox made it 5-3 in the eighth on Darnell McDonald's RBI single.
Notes
Branyan has homered in 36 of the 39 ballparks in which he's played after connecting for the first time at Fenway. The three he hasn't homered in are Target Field in Minnesota, which opened this year, Nationals Park in Washington, which opened in 2008, and Veterans Stadium in Philadelphia, where the Phillies played from 1971 to 2003. … The doubleheader was the first for both teams this year.