The Portuguese conquest of French Guiana, also known as Conquest of Cayenne, was a military operation against Cayenne, capital of the South American colony of French Guiana, launched in January 1809 in the context of the Napoleonic Wars. The invasion was undertaken by a combined expeditionary force that included Portuguese and British military contingents.

Dom John VI, nicknamed "the Clement", was King of the United Kingdom of Portugal, Brazil and the Algarves from 1816 to 1825. Although the United Kingdom of Portugal ceased to exist de facto beginning in 1822, he remained its monarch de jure between 1822 and 1825. After the recognition of the independence of Brazil under the Treaty of Rio de Janeiro of 1825, he continued as King of Portugal until his death in 1826. Under the same treaty, he also became titular Emperor of Brazil for life, while his son, Emperor Pedro I, was both de facto and de jure the monarch of the newly independent country.