“Jiang Tianyong had for a long time come under the influence of anti-Chinese infiltration, and gradually formed the idea of overturning the country’s existing political system,” said the decision from the court, which, like others in China, answers to the Communist Party. Without offering details, the court described Mr. Jiang as an agent for foreign forces hostile to the party’s rule.

The most specific charge against Mr. Jiang was that he had engaged in subversion by helping publicize the claims of another detained lawyer, Xie Yang, who said he had been tortured in custody, according to documents his lawyers shared early this year. Mr. Xie later retracted his claims at a trial in May that his wife and other supporters said was a show hearing.

Mr. Jiang, 46, had taken up contentious legal causes for more than a decade, including on behalf of members of Falun Gong, a banned spiritual sect. His clients included Chen Guangcheng, the human rights activist who fled to the American Embassy in Beijing before moving to the United States.

The government took away Mr. Jiang’s license to practice law in 2009, but he kept supporting dissidents and activists, even after July 2015, when the Chinese government initiated a wide clampdown. He was taken into custody last November, and at least part of his time in pretrial detention may be counted as part of his prison term. The court announcement did not specify a date of scheduled release.

Another activist detained and tried in the crackdown is still waiting for his verdict. Wu Gan, widely known by his internet handle “Super Vulgar Butcher,” stood trial on subversion charges in August. That hearing was not broadcast, and the court has yet to announce a decision.