History
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.

In 1916 the flag was changed to show a white elephant in royal regalia. In 1917, the current design, but with the middle colour being the same red as the outer stripe, was defined as the civil ensign. The story goes that during a flood King Vajiravudh (Rama VI) saw the flag hanging upside-down, and to prevent this from happening again created a new flag which was symmetrical. Later in 1917 the middle colour was changed to dark blue, which was similar in tone to indigo, which at the time was regarded as the auspicious colour for Saturday, the day King Vajiravudh was born. According to other sources, the blue colour was also chosen to show solidarity with the Allies of World War I, which also had the colours blue-red-white in their flags.

The flag resembles the flag of Costa Rica, which was adopted 11 years prior to Thailand’s. The main difference is that the blue and red colours are inverted; the flag of Costa Rica also has a different proportion of 3:5.
In the “reimagined” Battlestar Galactica television series in 2004, the color and layout of the flag of Caprica, one of twelve colonies in the series, is closed resemble to Thailand’s flag.