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 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 "legally providing information to outsiders", Li said, adding: "He provided information to foreign media about events in Xinjiang".
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.
