Lopez has repeatedly declared himself innocent of the charge of inciting violence at anti-government protests in 2014, calling himself a political prisoner.
"This is certainly a political trial. Unfortunately, the government's interest takes priority over the justice system," Lopez's lawyer Juan Carlos Gutierrez told AFP.
"They upheld his sentence under the same terms," Gutierrez said, referring to the 14-year sentence.
Riots hit Venezuela as food runs short. Play! 01:21
A 45-year-old US-educated economist, Lopez was arrested in February 2014 amid a wave of pro- and anti-government protests that swept Venezuela and left 43 people dead.
The government accused Lopez, one of leftist President Nicolas Maduro's most hardline opponents, of inciting the violence.
His sentence, handed down in September after a closed-door trial, was strongly condemned by the United Nations, the United States and the European Union.
During his 12-hour July appeals hearing, Lopez once again declared himself innocent.
Jesus Torrealba, spokesman for the opposition Democratic Unity Roundtable (MUD), wrote on Twitter Friday that the group rejects "the illegal and unjust condemnation of our brother @leopoldolopez and we reiterate that he will be freed by a vote of the people."
Venezuela's President Nicolas Maduro Credit: Marco Bello/Reuters
Venezuela's opposition has steeled itself in a battle to remove Maduro in a recall referendum, blaming the leader for the country's mounting chaos.
Once-booming Venezuela, home to the world's largest oil reserves, is gripped by severe shortages of food, medicine and basic goods that have triggered violence and looting.
The opposition is racing to force a referendum by January 10, four years into the president's six-year term.
Any time after that, a successful recall vote would simply transfer power to Maduro's hand-picked vice-president rather than trigger new elections. ()