Nangklao (Thai: นั่งเกล้า) or Rama III was the third monarch of Siam under the House of Chakri, ruling from 21 July 1824 to 2 April 1851. He succeeded his father, Rama II, as the King of Siam. His succession was unusual according to the traditions because Nangklao was a son of a concubine rather than a queen. His accession was perceived by foreign observers as having usurped the prior claim of Prince Mongkut, who was a legitimate son of Rama II born to a queen, Srisuriyendra. Under the old concept of Thai monarchy, however, a proper king must emulate Maha Sammata in that he must be "elected by the people". Ironically, Prince Mongkut may have later contributed to this misconception, when he feared that his own accession might be perceived by foreign observers as a usurpation.
During Nangklao's reign, the military hegemony of Siam was established by putting down the Laotian Rebellion (1826–1829, in what would come to be called Isan), the Siamese–Vietnamese War (1831–34), and the Siamese-Vietnamese War fought in Cambodia (1841–45).
Nangklao died on 2 April 1851 without having named a successor. He had 51 children including sons, but raised none of his consorts to queen. The throne passed to his half-brother, Prince Mongkut.