Lee sat nervously as the three-judge panel read the sentence at the Yueyang intermediate people's court in central Hunan province following a trial denounced by human rights groups as "unfair".
The court also deprived him of his political rights for two years.
A Chinese co-defendant named Peng Yuhua was sentenced to seven years in prison and he also lost his political rights for two years.
The court said both men stated that they would not appeal their sentences.
"This is an absolutely outrageous sentence that was handed out today, following on from an unfair trial," said Amnesty International China researcher William Nee.
"Lee Ming-cheh should not have to spend a day in jail, since everything he did -- peacefully discuss current events and historical issues on social media -- is expressly protected under international law," Nee told AFP.
Taiwan's presidential office and the ruling Democratic Progressive Party called the verdict "unacceptable".
China sees self-ruled Taiwan as part of its territory waiting to be reunified.
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