The flag of the Kingdom of Thailand (Thai: ธงไตรรงค์; RTGS: thong trai rong, meaning 'tricolour flag') shows five horizontal stripes in the colours red, white, blue, white and red, with the central blue stripe being twice as wide as each of the other four. The design was adopted on 28 September 1917, according to the royal decree issued by Rama VI. Since 2016, that day is a national day of importance in Thailand celebrating the flag.

The colours are said to stand for nation-religion-king, an unofficial motto of Thailand, red for the land and people, white for religions and blue for the monarchy, the last having been the auspicious colour of Rama VI. As the king declared war on Germany that July, some note the flag now bore the same colours as those of the UK, France, Russia and the United States.