Langdarma (Wylie: glang dar ma, THL: Lang Darma "Mature Bull" or "Dharma Bull", proper name U Dumtsen Wylie: 'u dum btsan) was the Tibetan Emperor, who most likely reigned from 838 to 841 CE. Early sources call him Tri Darma "King Dharma". His domain extended beyond Tibet to include Dunhuang and neighbouring Chinese regions.

By tradition Langdarma is held to have been anti-Buddhist and a follower of Bon. He is attributed with the assassination of his brother, King Ralpacan, in 838 CE and he is generally held to have persecuted Buddhists. According to traditional accounts, during the first two years of his rule, Langdarma remained a Buddhist, but under the influence of Wégyel Toré (Wylie: dbas rgyal to re), he became a follower of Bon.

The anti-Buddhist portrayal of this king has been questioned by several historians, most prominently Zuiho Yamaguchi.

Langdarma's reign was plagued by external troubles. The Uyghur Khaganate to the north collapsed due to a revolt by the Yenisei Kirghiz in year 840 and many displaced people fled to Tibet. According to one source, he only reigned for a year and a half, while others give six or thirteen years. According to traditional accounts, a Buddhist hermit or monk named Lhalung Pelgyi Dorje assassinated Langdarma in 842 or 846. His death was followed by civil war and the dissolution of the Tibetan empire, leading to the Era of Fragmentation.

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