Phra Bat Somdet Phra Poramentharamaha Vajiravudh Phra Mongkut Klao Chao Yu Hua or Phra Bat Somdet Phra Ramathibodi Si Sintharamaha Vajiravudh Phra Mongkut Klao Chao Yu Hua, or Rama VI (1 January 1881 – 25 November 1925) is the sixth monarch of Siam under the House of Chakri, ruling from 1910 until his death.

King Vajiravudh is known for his efforts to create and promote Siamese nationalism.

His reign is characterized by Siam's movement further towards democracy and minimal participation in the First World War.