Mamayewar Siamese na Kedah a 1821 wani gagarumin farmakin soji ne da Masarautar Siam ta kaddamar a kan Sarkin Musulmin Kedah, wanda ke a yankin arewacin kasar Malaysia a yau.A tarihi, Kedah ya kasance ƙarƙashin rinjayar Siamese, musamman a lokacin Ayutthaya.Koyaya, bayan faduwar Ayutthaya a cikin 1767, wannan ya canza na ɗan lokaci.Halin ya sake komawa lokacin da, a cikin 1786, Burtaniya ta sami hayar tsibirin Penang daga Sarkin Kedah don neman tallafin soja.A shekara ta 1820, tashin hankali ya karu lokacin da rahotanni suka nuna cewa Sarkin Kedah yana kulla kawance da Burma a kan Siam.Wannan ya jagoranci Sarki Rama II na Siam don ba da umarnin mamaye Kedah a 1821.An aiwatar da yakin Siamese kan Kedah da dabara.Da farko dai ba su da tabbas game da ainihin aniyar Kedah, Siamese sun tara manyan jiragen ruwa a ƙarƙashin Phraya Nakhon Noi, suna ɓarna ainihin manufarsu ta hanyar kai hari a wasu wurare.Lokacin da suka isa Alor Setar, sojojin Kedahan, wadanda ba su san da farmakin da ke tafe ba, suka yi mamaki.Wani gagarumin hari da aka kai ya kai ga kama wasu manyan mutanen Kedahan, yayin da sarkin ya yi nasarar tserewa zuwa Penang karkashin ikon Birtaniyya.Abin da ya biyo baya ya ga Siam ya tilasta wa Kedah mulki kai tsaye, yana nada ma'aikatan Siamese zuwa manyan mukamai da kuma kawo karshen wanzuwar sultan na wani lokaci.Sakamakon mamayewar yana da fa'ida mai fa'ida ta geopolitical.Birtaniya, sun damu da kasancewar Siamese da ke kusa da yankunansu, sun shiga tattaunawar diplomasiyya, wanda ya kai ga yarjejeniyar Burney a 1826. Wannan yarjejeniya ta amince da tasirin Siamese akan Kedah amma kuma ya gindaya wasu sharuɗɗa don tabbatar da bukatun Birtaniya.Duk da yarjejeniyar, adawa da mulkin Siamese ya ci gaba a Kedah.Sai bayan mutuwar Chao Phraya Nakhon Noi a shekara ta 1838 ne aka maido da mulkin Malay, inda daga karshe Sultan Ahmad Tajuddin ya sake samun karagar mulki a shekara ta 1842, duk da cewa karkashin kulawar Siamese.