Beltre's third multihomer game of the season and 19th of his career helped Seattle win for the seventh time in nine games.
The Indians entered the series with a 10-game winning streak but hit just .103 (3-for-29) with runners in scoring position over the three games against Seattle. Cleveland had not lost more than two games in a row since its season-worst 10-game slide from June 28-July 9.
"We had chances late again," Indians manager Eric Wedge said. "We were one hit away from tying or winning it in all three games. That was the story of the series."
In each instance, Seattle's bullpen came through instead of adding to its 25 blown saves.
Corcoran, the fourth reliever used by manager Jim Riggleman, walked three in the ninth but got Ryan Garko to ground into a game-ending double play.
"If they were horrible pitches, I would've been more concerned," Riggleman said of Corcoran's first eight pitches -- all outside the strike zone. "He wasn't missing that much. I was confident he would find the zone."
Corcoran deflected Garko's grounder with his bare hand. The ball bounced to shortstop Yuniesky Betancourt, who turned the double play.
"I thought I messed up," Corcoran said. "It probably was a sure double play, but I just reacted and touched it."
Ryan Rowland-Smith (4-2) allowed two runs, one earned, and seven hits over 6 1/3 innings to win his second straight start. The converted reliever struck out two and walked one in his seventh start in 42 outings this year.
"It's tough to sit and watch," Rowland-Smith said. "But the relievers did a great job."
Raul Ibanez went 1-for-3 with a run-scoring single for Seattle, and matched a career high for a single month with 31 RBIs in August. He hit .397 (44-for-111) with seven homers in 28 games this month.
Zach Jackson (0-1) gave up six runs and seven hits in five innings and Shin-Soo Choo hit his ninth homer for Cleveland. Jackson is one of four players acquired from Milwaukee in the trade that sent CC Sabathia to the Brewers in July.
Jackson hurt himself with a couple of mistakes in Seattle's five-run fifth. He hit Matt Clement in the back with a pitch with one out. Miguel Cairo then reached on a single when Jackson fielded his well-placed bunt and made an ill-advised sidearm throw past first baseman Ryan Garko for an error, sending the runners to second and third.
"That's a ball he needs to eat," Wedge said. "He had no play on that ball. It kind of dominoed from there."
Jackson said if given the chance, he would try again to make the play.
"I tried to make a good play and be aggressive and I threw it away," Jackson said. "I wouldn't have done anything different."
Ichiro Suzuki followed with an RBI single to give Seattle a 2-1 lead. Jackson tried to pick off Suzuki, but first baseman Martinez mishandled the throw for another error as Cairo scored from third. Ibanez singled in Suzuki and Beltre hit the next pitch over the wall for a 6-1 lead.
Seattle is 29-for-54 in save opportunities. ... The crowd cheered throughout the bottom of the ninth -- both for the Indians' odd rally and for the Blue Angels performing above the downtown skyline at the annual Cleveland Air Show. ... The Indians dealt minor league INF Jorge Velandia to Tampa Bay for cash. ... Indians CF Grady Sizemore was caught trying to steal third in the third. He had been successful on 28 of his previous 29 steal attempts. ... Seattle's 12 wins in August were its most in a month since going 12-15 in April. ... Ibanez also had 31 RBIs in July 2002.