The long-delayed pact has faced strong objections from opposition lawmakers. The ruling party reportedly called a special session of parliament to catch opposition members off guard and force the measure to a floor vote.
Shouts and screams filled the National Assembly as ruling party lawmakers forced their way onto the parliamentary floor.
Amid the scuffling, an opposition lawmaker fired a tear gas canister. He was quickly hustled out of the chamber.
South Korean lawmakers voted 151 to 7 in favor of ratifying the trade agreement. The pact is America's biggest free-trade agreement since the 1994 North American Free Trade Agreement with Canada and Mexico.
Two-way trade between the U.S. and South Korea totaled about $90 billion last year, according to the South Korean government.
The controversial vote touched off a small riot outside the National Assembly building. Hundreds of opponents of the deal scuffled with police mobilized to maintain order.
The protest had begun with a candlelight vigil, but it escalated into a full scale confrontation with police using water cannon.
Demonstrators demand that the trade bill be canceled, and they are also calling for South Korean President Lee Myung-bak to resign.
The Associated Press contributed to this report.