The Flag of Thailand shows five horizontal stripes in the colours red, white, blue, white and red, with the middle blue stripe being twice as wide as each of the other four. The three colours red-white-blue stand for nation-religion-king, an unofficial motto of Thailand. The flag was adopted on 28 October 1917. The Thai name for the flag is ธงไตรรงค์ (Thong Trairong), which simply means tricolour flag.

The first flag used for Siam was probably a plain red one, first used under King Narai (1656-1688). Naval flags later used different symbols on the red ground—a white chakra (the weapon of god Vishnu which use as the symbol of the House of Chakri), or a white elephant inside the chakra.

Officially the first flag was created in 1855 by King Mongkut (Rama IV), showing a white elephant (a royal symbol) on red ground, as the plain coloured flag was not distinct enough for international relations.