The Bảo Đại Thông Bảo (Hán tự: 保大通寶) was a round Copper-alloy coin with a square hole produced by the Nguyễn dynasty under French protection and was the last cash coin produced both in Vietnam and the world. The cast Bảo Đại Thông Bảo were produced by the Thanh Hóa Mint, while the machine-struck variants were produced in Hanoi by the colonial French government. These coins bear the name of Emperor Bảo Đại who ascended the throne in 1926 but continued the production of the earlier Khải Định Thông Bảo (啓定通寶) that bore his father's name until 1933 when he ordered the production of new coins with his reign name, which was normal as previous Vietnamese emperors also kept producing cash coins with the inscription of their predecessors for a period of time.
The Bảo Đại Thông Bảo were probably cast into 1941 or 1942 and the production was stopped because the occupying Japanese forces wanted the copper and were acquiring all of the cash coins they could find and stockpiling them in Haiphong for shipment to Japan for the production of war materials.
The French simultaneously began minting brass machine-struck coins in Hanoi with the same inscription as the cast Bảo Đại Thông Bảo coins that weighed 1.36 grams and had an official exchange rate of 1⁄600 piastre, but were probably only valued at 1⁄1000 piastre. These coins were being minted at the Hanoi Mint because the French had cut the funding for producing cast cash coins at the Thanh Hòa Mint which meant that the Protectorate of Annam wasn't producing enough cast cash coins to satisfy the demands of the Vietnamese markets for these low value coins for every day exchange.
There were two variants of this cash coins where one had a large version of the Chinese "大" (Đại) while the other had a smaller "大". After the Japanese had forced the production of Bảo Đại Thông Bảo coins because they were commandeering all copper in Vietnam new sapèque-like coins made from zinc were created in Hanoi and Osaka to replace these coins as the copper was being used for the production of Japanese weaponry and other military equipment, though the coins from Osaka didn't make it to the Vietnamese market as the shipping of war supplies was deemed more important by the Japanese government.
There were a total of 98,000,000 machine-struck Bảo Đại Thông Bảo sapèques produced.