Last year, Han Myung-sook was indicted on charges that she accepted $50,000 in kickbacks from a businessman seeking favors from her when she was in office in 2006. She and her main opposition Democratic Party have vehemently denied the allegations.

The Seoul Central District Court ruled Friday that allegations that the businessman gave her $50,000 were not proven, noting the businessman's testimony was questionable, said court spokesman Kim Sang-woo.

Prosecutors had asked for five years in prison.

Prosecutors can appeal the ruling in a week, though it remains unclear whether they will do so.

The ruling could give a boost to Han, who is seeking to run for Seoul mayor in local elections in June.

The opposition Democratic Party welcomed the ruling, saying the truth prevailed and her innocence was vindicated.

A spokesman for the ruling Grand National Party was not immediately reached for comment.

Han, served as South Korea's first female prime minister in 2006-2007 under former President Roh Moo-hyun, who committed suicide last year over a separate bribery scandal. She was one of Roh's close confidantes.

The position of prime minister is largely a ceremonial one in South Korea, where power is concentrated in the president's office.

In the 1970s, Han was jailed for two years under the then-authoritarian government for alleged involvement in pro-communist activities, but her work was later recognized as part of democratization efforts.