Protesters have held weeks of rallies against US beef imports in Seoul
The US has agreed not to export beef to South Korea from cattle older than 30 months in an effort to allay South Korean concerns over meat safety.
Both sides say the deal was reached during talks in Washington between top South Korean and US trade officials.
South Korea used to be a major market for US beef, but suspended imports in 2003 after a case of "mad cow disease" was identified there.
Seoul's decision in April to end the suspension sparked a wave of protests.
Speaking in Seoul on Saturday, Agriculture Minister Chung Woon-chun said the two sides had agreed "that beef from US cattle more than 30 months old should not be exported to South Korea until consumer confidence is restored".
The deal was later confirmed by US Trade Representative Susan Schwab, who held talks with South Korean Trade Minister Kim Jong-hoon this week.
The two countries will now introduce an age verification system.
On Friday, President Lee Myung-bak replaced several top aides following weeks of demonstrations over April's beef import deal - which was part of a landmark trade agreement.
The outcry has contributed to a sharp drop in popularity for the president, who won a landslide election victory last December.