The Burmese–Siamese War (1809–12) was an armed conflict fought between Burma and Siam, during the period of June 1809 and January 1812. The war centered over the control of the tin rich Tenasserim coast and served as the continuation of a long list of Burmese–Siamese wars. The conflict ended in a Siamese victory.

Control of the Tenasserim coast (present-day Mon State and Taninthayi Region in Myanmar) in the early 18th century was divided between Burma and Siam, with the Burmese controlling down to Tavoy (Dawei) and the Siamese controlling the rest. Throughout history, both kingdoms had claimed the entire coast – the Siamese to Martaban, and the Burmese to Junk Ceylon (or Phuket) – and control had changed hands several times. The Burmese Pagan Dynasty controlled the entire coast until 1287. Throughout the 14th and 16th centuries, Siamese kingdoms (first Sukhothai, later Ayutthaya) controlled much of the coast, up to just south of present-day Mawlamyaing. In the mid-16th century, the Burmese under Toungoo kings Tabinshwehti and Bayinnaung tried to regain the coast, first failing in 1548, and finally succeeding in 1564 when they conquered all of Siam for the first time. The Siamese revolted in 1584, and under their king Naresuan regained the lower coast by 1593 and the entire coast by 1594. The Burmese retook the upper coast down to Tavoy in 1614 but failed to recover the rest.

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