Norwegian national Mikael Davud, a member of China's Uighur minority and considered the mastermind behind the plot against the Jyllands-Posten daily, was first sentenced in January.
The Oslo appeals court last month upheld the guilty verdict against him and an accomplice, Shawan Sadek Saeed Bujak, an Iraqi Kurd residing in Norway.
The court, however, lopped six months off the three-and-a-half-year sentence originally meted out to Bujak.
Arrested in July 2010, the two men were found guilty of planning a bombing attack against the newspaper, which has been the target of several plots by Islamist extremists since publishing a series of inflammatory cartoons featuring Muhammad in 2005. One cartoon showed the prophet wearing a turban resembling a bomb with a lit fuse.
Prosecutors, who also accused the men of planning to murder cartoonist Kurt Westergaard, had initially demanded they receive prison sentences of 11 and five years respectively.
The court failed to come to a decision over the guilt of a third man, David Jakobsen -- an Uzbek arrested at the same time as Davud and Bujak.
Jakobsen was acquitted in January of the most serious charges but was sentenced to four months in jail for helping the others procure bomb-making material.
Though David acknowledged plotting an attack, he said he was targeting Chinese interests and not the newspaper. China's Uighur community is an oppressed minority.
He said he was acting for purely personal reasons and had manipulated the other two men so they could help him buy chemicals to make a bomb.