An appeals court sentenced Han Sang-gyun, head of the Korean Confederation of Trade Unions (KCTU), to three years in jail and fines of 500,000 won ($430), Tuesday, cutting his five-year prison sentence imposed by a lower court.
Han, 54, was arrested and indicted for leading a violent anti-government protest in November last year.
The Seoul High Court ruling has come amid recent peaceful rallies against President Park Geun-hye, where both protesters and police did not use violence.
Although the court reduced Han's sentence, it did not accept his claim that it was illegal for the police to set up barricades with police vehicles and use water cannons.
"The freedom of assembly should be guaranteed in a democracy, but this freedom is applied when the assembly is legal and peaceful," the court said. "But Han did not make an effort for a peaceful rally and rather instigated a clash with the police and prepared tools to destroy the vehicles."
However, the police's reactions to the protesters were "excessive," compared to the recent peaceful anti-President rallies, and the police suppression partly provoked violence from protesters, it ruled.
"In this time when a peaceful demonstration culture is being established, a long prison for Han does not contribute to the development of the culture," the court said.
For the last seven rallies in central Seoul, calling for President Park's resignation over the influence-peddling scandal, police set up barricades with their vehicles. But they were cautious and the protesters didn't move or destroy the vehicles.
Instead, people maintained peace and order, and the police, along with the rulings by the Seoul Administrative Court, allowed the protesters to get closer to Cheong Wa Dae gradually.
The prosecution earlier demanded eight years in jail, saying Han planned the massive violent rally, where farmer activist Baek Nam-ki was hit by a police water cannon and remained in a coma for 10 months before dying in September. After the rally, Han took refuge at Jogye Temple in central Seoul for almost a month.
The court also dismissed Han's charge of injury of police officers, saying the injuries were minor and did not require treatment.