Nangklao (พระบาทสมเด็จพระนั่งเกล้าเจ้าอยู่หัว, ; 31 March 1788 – 2 April 1851), birth name Thap (ทับ), also styled Rama III, was the third king of Siam under the House of Chakri, ruling from 21 July 1824 to 2 April 1851.
Nangklao was the eldest surviving son of his predecessor, king Rama II. His mother Sri Sulalai was one of the king's secondary wives. Nangklao was likely designated as heir by his father, his accession was uncontested and smoothly confirmed by the grand council. Foreign observers, however, falsely perceived him as having usurped the prior claim of his half-brother Prince Mongkut, who was younger, but born to queen Sri Suriyendra and thus "legitimate" according to Western customs. 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 (1826–1828, in what would come to be called Isan), the Siamese–Vietnamese War (1831–34), and the Siamese-Vietnamese War fought in Cambodia (1841–45).
King Nangklao was born as Prince Thap in 1788 to Prince Isarasundhorn and one of his royal wives Chao Chom Manda Riam, who came from a Muslim noble family from the south. Following Isarasundhorn's coronation (posthumously known as Phutthaloetla Naphalai, or Rama II) in 1809, Prince Kshatriyanuchit (Mom Men), the surviving son of Taksin, revolted as pretender to the throne. Prince Thap was assigned to suppress the rebellion, which he did. Praised by his father for his competence, Prince Thap was given the Sanskrit-derived title Chetsadabodin, raised to the bureaucratic rank of Kromma Muen, and served his father as Kromma Tha (minister of trade and foreign affairs.) As Kromma Tha, he developed proficiency in foreign trade, and developed an affection for Chinese goods and culture.