Thai nationalism is a political ideology involving the application of nationalism to the political discourse of Thailand. It was first popularised by King Vajiravudh, and was subsequently adopted and adapted by various leading political factions throughout the twentieth century.

Vajiravudh was the sixth king of Siam from the Chakri dynasty, titled Rama VI. He reigned from 1910 until his death in 1925. King Vajiravudh is best known for his efforts to create and promote Siamese nationalism. His reign was characterized by Siam's movement further towards democracy and minimal participation in World War I. He had keen interests in Siamese history, archaeology, and literature, as well as economics, politics and world affairs, and founded the country's first university, Chulalongkorn University.