The Seoul High Court upheld a lower court's decision in January that found Arthur Patterson, now 36, guilty of murdering Jo Jung-Pil, who was stabbed multiple times with a knife in a fast-food restaurant toilet.
"The defendant lived 19 years of life that the victim was deprived of but did not show any sign of sincere remorse seeking pardon from the victim and the bereaved family," South Korea's Yonhap news agency quoted the court as saying.
Twenty years in jail is the maximum penalty in South Korea for an offender under the age of 18.
Patterson, the son of a US military contractor, had denied any involvement in the crime which took place in the nightlife district of Itaewon, close to the US military base in Seoul.
More From This Section
He was initially tried in 1997 as an accomplice, while Edward Lee, a Korean-American man who was at the scene, was charged with murder. Both accused the other of killing Jo.
In 1998, Patterson was released as part of an amnesty programme, only to find himself a murder suspect again after Lee was acquitted on appeal for lack of evidence.
He fled to the United States a year later after investigators failed to renew his travel ban -- a mistake that sparked a storm of criticism.
The murder was made into a hit movie in 2009, reviving public anger over the crime and forcing South Korean prosecutors to reopen the case.
His lawyer said he would file another appeal against the verdict.