Nangklao (born Thap; 31 March 1788 – 2 April 1851), also known by his regnal name Rama III, was the king of Siam who initiated the country's first cautious engagements with the West. His reign marked the period during which Siam's territorial extent reached its greatest size.

Nangklao was the eldest surviving son of King Rama II. His mother Sri Sulalai was one of Rama II'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 younger half-brother Prince Mongkut, who was 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, 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 crown strengthened the tax-farming system, which became a major and more reliable source of revenue for the kingdom. The Burney Treaty (1826) further liberalized foreign trade and reduced the royal warehouse monopoly, generating substantial income for the state. For his contributions to trade and economic development, he is regarded as "the father of Thai commerce". In military affairs, Siam consolidated its regional dominance by suppressing the Laotian Rebellion (1826–1828, in what is now the Isan region) and by prevailing in the Siamese–Vietnamese War (1831–34) and the subsequent campaign in Cambodia (1841–1845).

Nangklao was born as Prince Thap in 1788 to Prince Itsarasunthon and one of his royal wives, Chao Chom Manda Riam, who came from a Muslim noble family from the south. Riam also birthed two siblings for Prince Thap who both died in the same year, Pom (1790–93) and Noo Dam (1792–93). Following Itsarasunthon's coronation (posthumously known as Phutthaloetla Naphalai, or Rama II) in 1809, Prince Kshatriyanuchit (Mom Men), a 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). He received the sobriquet "Chao Sua", bestowed by his father in recognition of his commercial acumen. As Kromma Tha, he developed proficiency in foreign trade, and developed an affection for Chinese goods and culture. Temples he later had constructed were characterized by Chinese influence. After a private audience in 1822, Crawfurd wrote of the Prince Krom-chiat that, "he seemed certainly to maintain the character assigned to him in public estimation, of being the most intelligent of all the princes and chiefs of the Siamese Court." The Portuguese Consul stated that the Prince had offered him a large sum of money, if he would translate from the French into the Portuguese language a history of the wars of Napoleon, for the purpose of being rendered into Siamese through the Christian interpreters.

As the prince administered trade affairs, his half-brother Prince Mongkut pursued the way of religion, becoming a monk in 1824. In that year, Phutthaloetla Naphalai died suddenly without having named a successor to viceroy Maha Senanurak, who had died 16 July 1817. According to the traditions of royal succession, the viceroy or uparaja was heir presumptive. If there were none, then an ad hoc senabodi consisting of senior officials present at the death of a king, would elect a successor. Foreign observers accustomed to the concept of an heir apparent expected Prince Mongkut, as a son of the queen, to ascend to the throne. However, the assembled Senabodi considered Prince Chetsadabodin a more competent choice as he had served the king in Kromma Tha for years. Support came strongly from high-ranking nobility, including Chao Phraya Abhay Pudhorn, the Samuha Nayok, and Dis Bunnag then Minister of Kromma Tha, and other Bunnag family members.

Chetsadabodin accepted the throne and was crowned in 1824. He raised his mother Riam to Princess Mother Sri Sulalai. He appointed his uncle, Sakdiphonlasep, viceroy on 21 July 1824 – who predeceased the king 1 May 1832, leading to yet another succession crisis. He did not name his reign, but was posthumously awarded the name Nangklao by Mongkut, who had in the interim remained in ecclesiastic status to avoid the intrigues of royal politics.

The reign of Nangklao (as he was posthumously known) saw the renewal of Western contacts. In 1822, British East India Company agent John Crawfurd's mission to Siam laid the groundwork for a British request for Siamese support in the First Anglo-Burmese War, which broke out in 1824. Nangklao provided fleets and elephants and sent Siamese armies to participate in the invasion of Burma since the British promised Siam the conquered lands. Phraya Chumporn ordered the forced migration of Mergui (a common practice in Southeast Asia regarding the newly-conquered lands), which had been conquered by the British. The British were frustrated at Phraya Chumporn's actions, and hostilities were heightened. Nangklao ordered the Siamese armies to leave to avoid further conflict.^{[citation needed]}

In 1825, Henry Burney arrived to negotiate peace agreements. The Burney Treaty was the first treaty with the West during the Rattanakosin period. Its purpose was to establish free trade in Siam and to greatly reduce taxation on foreign trading ships. Whether it accomplished these objectives is disputed.