The flag of Tonga consists of a red field with a white canton charged with a red couped cross. Adopted in 1875 after being officially enshrined into the nation's constitution, it has been the flag of the Kingdom of Tonga since that year. The constitution stipulates that the national flag can never be changed.

The British first arrived in Tonga in the late-18th century, when Captain James Cook made three visits to the islands between 1773 and 1777. Approximately fifty years later, English Wesleyan Methodist missionaries came to Tonga and began converting the islanders to Christianity. In 1831, they succeeded in converting "paramount chief" Taufa'ahau Tupou, who became King George Tupou I in 1845. It was during this time (circa 1840s) that the first Tongan flag was adopted. It consisted of a white field with a cross (either red or blue in colour) at all four corners, and the letters "A" (in red) and "M" (in blue) at the centre that symbolise the king.

Upon his accession to the throne, the king sought to design a new flag for the nation, one that would represent Christianity. He befriended Shirley Waldemar Baker – a member of the United Kingdom's Tongan mission who later became the Prime Minister of Tonga – and they worked together to formulate a new flag, coat of arms and national anthem for Tonga. The new design resembled the British Red Ensign, in that three-quarters of it consisted of a simple red field, with a "distinctive canton" featured in the upper hoist section; this was first used in 1866. A new constitution for the kingdom was formulated and proclaimed on 4 November 1875. It codified the new flag design, and marks when it was adopted as the national flag. Under Article 47 of the Constitution, the flag can "never be altered" and "shall always be the flag" of Tonga.