The flag looks similar to the flag of the Red Cross. The flag was originally identical to that flag, but to avoid confusion, it was changed so that the red cross appeared as a canton of a red ensign, making it similar to the 17th-century English Red Ensign.

The flag has been in use since 1864 but it was officially adopted only in 1875. Clause 47 of the Tongan Constitution states: "The Flag of Tonga shall never be altered but shall always be the flag of the Kingdom."