A court in the regional capital Urumqi found Zhang Haitao guilty of “incitement to subvert state power” and “illegally providing intelligence overseas”, his lawyer Li Dunyong told AFP.
China regularly jails dissidents who have spoken out against the Communist Party, but observers said the sentence handed down on Sunday was unusually harsh.
Xinjiang, home to the mainly Muslim Uighur minority, has in recent years seen a security crackdown prompted by clashes in the region that have killed hundreds.
China says it faces a terrorist threat, while rights groups say restrictions on Islam have created ethnic tensions and that police violence often goes unreported.
China tightly limits domestic media from publishing information about Xinjiang, and foreign journalists are regularly obstructed while reporting there.
The 15-year sentence for “inciting subversion” – the maximum possible for the charge – was “about some postings (Zhang) made online… they were about government policies, mainly in Xinjiang,” Li said.
“The postings advocated democracy, and opposed dictatorship,” he added. “The things he did don’t constitute a crime”.
Zhang received an additional four year sentence on a charge of “illegally providing information to outsiders”, Li said, adding: “He provided information to foreign media about events in Xinjiang”.
Maya Wang, a China researcher at Human Rights Watch, told AFP: “It’s quite unusual to have that maximum sentence”.
The verdict is more than the 11-year jailing given to writer Liu Xiaobo in 2009 on the same charge, over a petition calling for democratic reforms. Liu was awarded the Nobel Peace Prize a year later.
The Communist Party does not tolerate open challenges to its right to rule, and President Xi Jinping has overseen a crackdown on activists since coming to power in 2012.
A court in Urumqi jailed Uighur academic Ilham Tohti for life for “separatism” in 2104 over a website he ran that was often critical of official ethnic policies.
Zhang’s sentence comes in the same month that China formally arrested more than 10 human rights lawyers and legal staff on “state-subversion” related charges.
The Xinjiang court declined to comment when contacted by AFP.
Zhang’s lawyer Li added: “The sentence is very heavy. But in Xinjiang verdicts are always stronger than in the rest of the country”.