Both houses of Congress voted in quick succession on Wednesday to approve the agreement, as well as pacts with Panama and Colombia.
It will now go to President Barack Obama to be signed into law.
The agreement is expected to increase US exports to the Asian economy by as much as $10bn (Â£6.5bn).
The free-trade deal with South Korea is the largest US trade pact since it signed the North American Free Trade Agreement in 1994.
However, it still needs to be passed by the South Korean parliament.
'Made in America'
President Obama said the pacts with South Korea, Panama and Colombia were "a major win for American workers and businesses".
"Tonight's vote, with bipartisan support, will significantly boost exports that bear the proud label 'Made in America', support tens of thousands of good-paying American jobs and protect labour rights, the environment and intellectual property," he said in a statement.
The Obama administration says that pact alone will support 70,000 American jobs.
There was nearly $80bn in trade between the US and South Korea last year. The Asian country is the seventh largest trade partner for the US.
The vote coincided with South Korean President Lee Myung-bak's arrival in Washington for an official visit.
In a speech on Wednesday he said the agreement would "send a powerful message to the world that the US and South Korea stand together in rejecting protectionism and that we are open to free and fair trade".
The pact was first agreed in 2007, but concerns in the US over tariffs imposed by South Korea on US carmakers delayed the proceedings.

Media playback is not supported on this device
He lost the captaincy to Alun Wyn Jones at the start of 2017.
"He's been outstanding in the first couple of games in the way that he's competed (at the break-down) and brought physicality," said Gatland.
Warburton has been playing at blind-side flanker alongside Justin Tipuric.
Gatland says Warburton, the Lions captain in 2013 in Australia, has shown the sort of attributes that will be needed on the 2017 tour to New Zealand.
"New Zealand have got three or four open-sides that are very strong at the break-down and you've got to be able to have someone who can do that," said Gatland.
"I'm not saying you don't have someone at seven who can carry and play a bit wider, there are options for that as well but you get potentially poor weather conditions in New Zealand and someone who's able to get on the ball, slow things down, Sam's able to do that.
"For him the challenge is to stay injury-free because he's picked up a few injuries, the challenge for him and a lot of players is getting a run of games under his belt."
But Gatland also highlighted the strength of Wales's next opponents Scotland in the same area of the pitch, after their narrow defeat to France in Paris, despite injuries to flankers John Barclay and John Hardie.
"The break-down area is pretty critical and I thought on Sunday it was one area Scotland were very strong against France, it kept them in the game because of the turnovers they got, the penalties and they competed very hard at the break-down, it was a real strength of the Scottish performance."
Former Ireland and Lions captain Brian O'Driscoll believes that Scotland open-side flanker Hamish Watson is currently worth a place in the Lions match-day squad of 23, with their win over Ireland boosting their chances of improving their Lions representation.
"I've been really impressed with Watson at seven, the first couple of games, (lock) Jonny Gray's been great, (full-back) Stuart Hogg's had a quality couple of games," O'Driscoll told BBC Wales Sport.
"If they can pick up another victory at least, I think you'll have more of a showing from the Scottish team, (wing Tommy) Seymour as well has an outside chance, so maybe you'll have more than you've had in recent years."
US lawmakers have approved a long-delayed free trade agreement with South Korea, calling it the most significant in 16 years.
[NEXT_CONCEPT]
Wales flanker Sam Warburton has been praised as "outstanding" in the first two rounds of the Six Nations by British and Irish Lions coach Warren Gatland.