Lao–Siamese War or the Siamese Invasion of Laos (1778–1779) is the military conflict between Thonburi Kingdom of Siam (now Thailand) and the Lao kingdoms of Vientiane and Champasak. The war resulted in all three Lao kingdoms of Luang Phrabang, Vientiane and Champasak becoming Siamese tributary vassal kingdoms under Siamese suzerainty and domination in Thonburi and the subsequent Rattanakosin Period.

By 1779 General Taksin had driven the Burmese from Siam, had overrun the Lao Kingdoms of Champasak and Vientiane, and forced Luang Prabang to accept vassalage (Luang Prabang had aided Siam during the siege of Vientiane). Traditional power relationships in Southeast Asia followed the Mandala model, warfare was waged to secure population centers for corvee labor, control regional trade, and confirm religious and secular authority by controlling potent Buddhist symbols (white elephants, important stupas, temples, and Buddha images). To legitimize the Thonburi Dynasty, General Taksin seized the Emerald Buddha and Phra Bang images from Vientiane. Taksin also demanded that the ruling elites of the Lao kingdoms and their royal families pledge vassalage to Siam in order to retain their regional autonomy in accordance with the Mandala model. In the traditional Mandala model, vassal kings retained their power to raise tax, discipline their own vassals, inflict capital punishment, and appoint their own officials. Only matters of war, and succession required approval from the suzerain. Vassals were also expected to provide annual tribute of gold and silver (traditionally modeled into trees), provide tax and tax in-kind, raise support armies in time of war, and provide corvee labor for state projects.