Their third walk-off defeat of the year dropped them a season-low 19 games under .500, coming on a sacrifice fly with the bases loaded by John Jaso. The game-winner came after first baseman Justin Morneau had trouble fielding a bunt that put two runners on with nobody out.
"And at the end, we miss a play, and that always comes back to get you," Twins manager Ron Gardenhire said. "Justin came up and didn't get the ball out of his glove, and we didn't get the out at first base, and that sets up the whole inning. We kind of shot ourselves in the foot that inning more than anything else."
It started when left-handed reliever Tyler Robertson gave up a leadoff single to Michael Saunders before Brendan Ryan laid down a bunt down the first-base line. Morneau fielded it cleanly, but hesitated before throwing to first to allow Ryan to reach on what was ruled a base hit.
Robertson then walked Dustin Ackley to set the stage for Jaso's pinch-hit sacrifice fly to left field off right-handed reliever Casey Fien.
"He's going to stick his nose in there, he's not going to give anything away," Mariners manager Eric Wedge said of Jaso. "You feel good with him up there in those situations."
It spoiled a strong effort from Diamond, who gave up just two runs on nine hits over 6 2/3 innings, but was saddled with a no-decision. He was in line for the win before giving up a game-tying homer to Justin Smoak to lead off the seventh.
"I feel like that loss falls on that pitch right there," Diamond said. "He really got into it. I was a little frustrated, I think, because I thought I had him on that [previous] curveball. I let that get the best of me."
The Twins got on the board against fellow left-hander Jason Vargas in the first, scoring on an RBI single by Morneau with two outs.
"They were able to squeeze some balls in there," said Vargas, who allowed two runs on eight hits over 6 2/3 innings. "And they were just able to make me grind that first inning."
The Mariners came back to tie it at 1-1 in the third on an RBI single by Ackley, after back-to-back singles from Saunders and Ryan.
Minnesota retook the lead in the seventh, when Joe Mauer beat out a potential double play with the bases loaded and one out.
But the lead was short-lived, as Seattle scored in the bottom half of the frame on Smoak's solo shot to tie it at 2-2. It was his 14th of the year and his first since July 17.
The Twins had a chance to take a lead in the ninth, with the bases loaded and two outs, but Josh Willingham flied out to right off closer Tom Wilhelmsen to end the inning.
It was another wasted opportunity for the Twins, who went 2-for-12 with runners in scoring position to leave 11 men on base.
"Unfortunately we couldn't come up with a hit," Gardenhire said. "We missed so many opportunities with runners in scoring position. We left a few too many out there. It happens in this game, but we ended up coming up with a loss."