The Flag of former South Vietnam was designed by Emperor Thành Thái in 1890 and was used by Emperor Bảo Đại in 1948. It was the flag of the former State of Vietnam (the French-controlled areas in both Northern and Southern Vietnam) from 1949 to 1955 and later of the Republic of Vietnam (South Vietnam) from 1955 until April 30, 1975 when the south unconditionally surrendered to the north, where it was officially joined in a unified Vietnam a year later. The flag consists of a yellow field and three horizontal red stripes to and can be explained as either symbolising the unifying blood running through northern, central, and southern Vietnam, or as representing the symbol for "south" (as in, south from China), in Daoist trigrams.

It is still used by some Vietnamese immigrants now living in other countries. From February 2003 to January 2006, in the United States, nine states, three counties and 76 cities have adopted resolutions recognizing the yellow flag as the "Vietnamese Heritage & Freedom Flag".