The Siamese invasion of Kedah in 1821 was a significant military operation launched by the Kingdom of Siam against the Sultanate of Kedah, situated in today's northern Peninsula Malaysia. Historically, Kedah had been under Siamese influence, especially during the Ayutthaya period. However, after the fall of Ayutthaya in 1767, this changed temporarily. The dynamics shifted again when, in 1786, the British acquired a lease of Penang Island from Kedah's sultan in return for military support. By 1820, tensions escalated when reports suggested that the sultan of Kedah was forming an alliance with the Burmese against Siam. This led King Rama II of Siam to order an invasion of Kedah in 1821.

The Siamese campaign against Kedah was strategically executed. Initially uncertain about Kedah's true intentions, the Siamese amassed a significant fleet under Phraya Nakhon Noi, disguising their true intent by feigning an attack on other locations. When they reached Alor Setar, the Kedahan forces, unaware of the impending invasion, were taken by surprise. A swift and decisive attack led to the capture of key Kedahan figures, while the sultan managed to escape to British-controlled Penang. The aftermath saw Siam impose direct rule over Kedah, appointing Siamese personnel to key positions and effectively ending the sultanate's existence for a period.

The repercussions of the invasion had broader geopolitical implications. The British, concerned about the Siamese presence so close to their territories, engaged in diplomatic negotiations, leading to the Burney Treaty in 1826. This treaty recognized Siamese influence over Kedah but also set certain conditions to ensure British interests. Despite the treaty, resistance against Siamese rule persisted in Kedah. It was only after the death of Chao Phraya Nakhon Noi in 1838 that Malay rule was restored, with Sultan Ahmad Tajuddin finally regaining his throne in 1842, albeit under Siamese oversight.