Tianjun County, in Tibetan Themchen, is a county of Qinghai Province, China, bordering Gansu Province to the north.

It is under the administration of the Haixi Mongol and Tibetan Autonomous Prefecture.

Tianjun County is divided into 3 towns and 7 townships.

The county is served by the Qinghai–Tibet Railway; its Tianjun railway station is located near the county seat, Xinyuan Town.

Large coalfields are worked near Muli Town in the northern part of the county, at the elevation around 4,000 m above the sea level.