The appellate court upheld the lower court's finding that Han engaged in key acts to help Yoon declare martial law and did not dissuade him.
Han, 76, convened a cabinet meeting to facilitate legal procedures to impose martial law, the judge said, rejecting his claim that the gathering was intended to block Yoon.
The judge said Han was also aware of the former safety minister's plan to cut off supplies of power and water to some media companies during martial law, but failed to prevent it.
"The defendant, as prime minister, had the responsibility to curb and control the president's wrongful execution of his power and help him legally exercise his authority," the lead judge said.
"But he abandoned the enormous duty and joined the side participating in insurrection .... He committed crimes afterwards to cover up his guilt, so his crime is grave."
The court also found him guilty of fabricating a presidential document and perjury.
The judge, however, said he took into account Han's 50-year-long career in public service, including posts at the finance ministry and foreign ministry.
It was also not clearly established that Han actively participated in Yoon's plan, the judge said, noting how he later convened another cabinet meeting to rescind martial law after parliament voted it down.
A lower court had in January handed Han a 23-year jail term, saying he "turned a blind eye" to the martial law plan and engaged in a key action of insurrection.
Prosecutors, who had requested a 15-year prison sentence ahead of the first ruling, sought 23 years for the appeal.
Han denied wrongdoing on all charges except perjury, saying in November that while he regretted not being able to stop Yoon from declaring martial law, he "never agreed to it or tried to help."
His lawyer said he would appeal again.
Yoon, who faces eight separate trials, was sentenced to life in prison on the most serious charge of leading an insurrection tied to his declaration of martial law. He has denied wrongdoing.
