A resident of the area found the 24-year-old woman floating unconscious and rescued her, police said.
She told police she jumped off a bridge Sunday morning with another woman and three men she met on the internet, said the police, who are searching for the four others.
They had already tried and failed to kill themselves at a nearby motel room, a police detective on the case told the AFP news agency.
Suicide is the leading cause of death among young people in South Korea with heavy stress thanks to its highly competitive educational environment and job market.
The country has one of the world's highest suicide rates.