The United States, European Union and international rights groups had called for the release of the monk, Tenzin Delek Rinpoche, 65, who died on Sunday in Chuandong prison in the southwestern city of Chengdu, his cousin, Geshe Nyima, told Reuters by telephone from India.
Tenzin Delek was serving a 20-year sentence on charges of "crimes of terror and incitement of separatism".
The cause of death was not clear, Geshe Nyima said, adding that authorities had notified Tenzin Delek's sisters on Sunday but did not elaborate on the circumstances of his death.