Josh Willingham hit a solo homer off Jon Rauch to lead off the 10th inning, and the Nationals fell, 6-5, in front of a crowd of 10,888.
Washington is now 1-9 against the Marlins this season.
"So far this team has had our number, and we've been unable to keep them in the ballpark," said manager Manny Acta about the Marlins, who lead the Major Leagues in home runs. "That's been the story for two years. They just bomb away, and they just seem to do it against us constantly."
Hours after finding out that closer Chad Cordero would miss the rest of the season because of a torn labrum, the man who took his place, Rauch, blew his fifth save of the season in the ninth and took his second loss in the 10th.
In a 5-4 game with one out in the bottom of the ninth, Hanley Ramirez got a 2-2 fastball from the 6-foot-11 right-hander and sent it over the left-field scoreboard. It was the second straight night that Ramirez hit a tying homer.
Then, after Wil Nieves struck out with runners on the corners and two outs in the top of the 10th, Willingham came up in the bottom of the frame and sent a first-pitch, chest-high slider out to left-center for his second homer of the night.
"One of those things about my job is if I come in, it generally costs us ballgames, and that's what happened tonight," said Rauch, who entered the game having a stellar year, going 4-1 with a 2.18 ERA and a career-high 16 saves. "[I have to] forget about it. I can't do anything about it now. I can't take those pitches back. I could make better pitches tomorrow, and the day after, and the day after.
"That's all I'm worried about."
Tim Redding has now taken a no-decision in eight consecutive starts. The 30-year-old right-hander finished the night giving up four runs on eight hits while striking out six and walking two. He left after throwing 111 pitches in five innings, his shortest outing since April 23.
"I could have commanded the zone a little bit more, but I just didn't have my best stuff -- I just tried to battle the best I could," Redding said. "I wanted to go out there for the sixth inning, despite the pitch count. I thought I had it in me ... but Manny has to protect the game. He made the right move, and it just slipped away."
The loss came in a seesaw game that was tied three times and saw the lead switch hands on four occasions.
After the Marlins tied the game on a sacrifice fly by Matt Treanor in the sixth, the Nationals retook the lead in the top of the eighth on a wild pitch. Going into that inning, the Nats had been burned twice on hit-and-run plays, but they got a little revenge when they put the speedy Ronnie Belliard in motion and Paul Lo Duca -- starting in left field -- grounded to where the shortstop Ramirez was supposed to be, putting runners on the corners and allowing Belliard to score on a wild pitch by Renyel Pinto, who had just come out of the bullpen.
After the Marlins opened the scoring on an RBI single by Robert Andino in the second, Cristian Guzman tied the game on a two-out base hit to right field in the top of the third. The Marlins then made it a two-run deficit off Willingham's first home run -- a two-run shot in the third -- but the Nationals' offense didn't go quietly.
Willie Harris made it a one-run game with a single up the middle in the fourth inning. Then, in the fifth, Elijah Dukes hit a towering drive to left field to tie the game. Two batters later, Dmitri Young went deep to right-center field off Marlins starter Ryan Tucker -- who allowed four runs on six hits in five innings -- to give the Nationals the lead.
Jesus Colome and Saul Rivera combined to pitch three shutout innings before the Nationals' most consistent reliever all season let the game slip.
"I want [Rauch] in that same situation again tomorrow," Acta said. "[The Nationals] played hard, and that's what I liked.
"I'm proud of the way they played."