Tibet's cultural heritage suffered during the chaos of the 1966-79 Cultural Revolution, when fanatical Maoist Red Guards smashed religious symbols and destroyed monasteries. Some have since been rebuilt.
The decree on artifacts is part of revised rules on protecting Tibet's cultural heritage, Xinhua said.
"Relics collected or taken care of in places of religious activity in the autonomous region, or provided for the use of religious personnel by national artifact collection departments, belong to the state," Xinhua quoted the revised rules as saying.
That rule was needed due to the "special situation of preserving the existence of Tibet's relics," the report added, without elaborating.
China has run Tibet with an iron fist since People's Liberation Army troops occupied it in 1950, and has vowed to promote economic prosperity.
Tibetans who dare to protest or demand greater freedoms are often imprisoned and may be tortured, rights groups say.
The new rules also stipulate that all explanatory signs at cultural sites such as tombs, grottos and cave paintings must be in Tibetan as well as Chinese.
The Tibetan language is meant to have equal status with Chinese in Tibet, although in practice few non-Tibetans understand it and many complain it is being marginalized.