A former South Korean student activist at the centre of a notorious miscarriage of justice in the era of military rule was acquitted Thursday in a retrial of abetting a fellow activist's suicide.

Kang Ki-Hoon was convicted in 1992 of aiding activist Kim Ki-Sul to set himself on fire and leap to his death from a university building in protest at the military-backed administration of then president Roh Tae-Woo.

He was also found guilty of forging Kim's will and was sentenced to three years in prison.

The case became known as South Korea's "Dreyfus Affair" with Kang, now 51, widely considered to have been framed by Roh's government in order to discredit the growing protest movement.

After serving his prison term, Kang, who always denied the charges, sought a retrial, which he was finally granted under the auspices of a Truth and Reconciliation Commission set up in 2005 to review past rights abuses.

Delivering its not guilty verdict on Thursday, the Seoul High Court said forensic evidence produced at the time by government prosecutors to show Kang wrote Kim's will "lacked credibility".

Kim's self-immolation in May 1991 was the fourth in less than two weeks amid nationwide protests demanding Roh's resignation and broader democratic reforms.

The Truth and Reconciliation Commission had already ruled Kang's conviction a miscarriage of justice back in 2007, but a retrial was required to give the ruling legal weight. (AFP)