Phra Bat Somdet Phra Poramintharamaha Chakri Borommanat Phra Buddha Yodfa Chulaloke (Thai: พระบาทสมเด็จพระปรมินทรมหาจักรีบรมนาถฯ พระพุทธยอดฟ้าจุฬาโลก; RTGS: — Phra Phuttha Yot Fa Chula Lok), posthumously titled "the Great", or Rama I (20 March 1736 – 7 September 1809), was the founder and the first monarch of the reigning House of Chakri of Siam (now Thailand). He ascended the throne in 1782, after defeating a rebellion which had deposed King Taksin of Thonburi. He was also celebrated as the founder of Rattanakosin (now Bangkok) as the new capital of the reunited kingdom.

Rama I was born in the Kingdom of Ayutthaya, and had served King Taksin in wars against the Burmese Konbaung dynasty and helped him in the reunification of Siam. During this time he emerged as Siam's most powerful military leader. In 1782, he took control of Siam and crowned himself as the monarch. The most famous event in his reign was the Burmese - Siamese War of 1785, which was the last major Burmese assault on Siam. Buddha Yodfa Chulaloke was also the first Somdet Chao Phraya, the highest rank the nobility could attain, equaled to that of royalty.

Thong Duang was born in 1736 in the reign of King Boromakot of Ayutthaya. His father was Thong Dee (later raised as Somdet Phra Prathom Borom Maha Rajchanok – the grand primordial father) who was "Phra Aksara Sundhornsat" (Royal Secretary of northern Siam, Keeper of the Royal Seal). Aksara Sundhornsat was also a descendant of Kosa Pan, the leader of King Narai's embassy to the French court, and was of Mon descent. His mother, Daoreung (original name Yok), was part Chinese. Thong Duang had six other siblings.

Thong Duang at a young age entered the Royal Palace as one of the royal pages of King Uthumporn, where he met his childhood friend Taksin. In 1757, aged 21, he became a monk temporarily, in accordance with Siamese custom. In 1760, he married Nak, daughter of a town patron in Samut Sakorn. He was later appointed the Luang Yokkrabat (Governor of) Ratchaburi by King Ekatat in 1758.

On the eve of the fall of Ayutthaya, Phraya Wachira Prakarn (later King Taksin) had foreseen that the fall of the city was certain. Wachira Prakarn decided to break the siege of the city of Ayutthaya by the Burmese army and establish a new base outside. Phraya Ratchaburi also joined this venture. In 1767, Ayutthaya under King Ekatat fell to Burmese invaders, the city was completely destroyed; burned and looted. Local warlords rose up to establish their supremacy in the absence of a central authority. Despite the fall of Ayutthaya, Taksin and his men in the same year managed to capture Chantaburi and Trat. During this time Phraya Ratchaburi became one of Taksin's six ministers and together with Phraya Pichai they were regarded by Taksin as his two most valuable generals.

Swiftly Taksin made a strategic plan and under it recaptured Ayutthaya in one year. In 1768 Taksin crowned himself and founded the Kingdom of Thonburi on the west bank of the mouth of the Chao Phraya river, using Thonburi as a new capital. Under the new Thonburi regime, Thong Duang was appointed Phra Raja Warindra (Royal Police). After subjugating the warlord of Pimai with his brother Maha Montri (later Maha Sura Singhanat), he was raised to Phraya Abhaya Ronarit.

After the campaign to subdue the lord of Fang in 1769, Abhaya Ronarit was raised to Phraya Yommaraj and in the next year became Chao Phraya Maha Chakri – the Samuha Nayok (Prime Minister). Maha Chakri joined the Burmese wars and went on to subjugate Cambodia. His brother, Phraya Anuchit Raja (previously Maha Montri), accompanied him in various campaigns. Chakri and his brother Phraya Surasi was sent to the north to Lanna in 1774 to free the kingdom from Burmese rule with the help of Kawila, a prince from Lampang. In 1776, he conquered Khmer Pa Dong (around modern Surin). He was assigned the task of conquering Laotian kingdoms in 1778 and all the three kingdoms (Vientiane, Luang Prabang, Champasak) fell to the Siamese in the same year. He was eventually raised to Somdet Chao Phraya Maha Kshatriyaseuk, the first Somdet Chao Phraya.

In 1781, Chao Phraya went on the campaigns against Cambodia, only to return prematurely due to the instability of Thonburi. The rebellion of Phraya San had broken out and the rebels deposed King Taksin. Some sources report that Taksin was consigned to a monastery. After arriving in Thonburi in 1782, Chao Phraya defeated the Phraya San with his forces. Later sources widely reported that the general eventually executed the ousted Taksin, contradicting some earlier sources. He then seized power and made himself King, establishing the Chakri Dynasty, which continues to rule Thailand to this day.

General Maha Kshatriyaseuk crowned himself on 6 April 1782. Without naming himself (he was only referred to as King or His Majesty), he was later given his name as Phrabat Somdet Phra Buddha Yodfa Chulaloke or Rama I or the First Reign much later by his descendants . Rama I decided to move the capital of Siam to the east bank of the Chao Phraya river for several reasons, including its better strategic location and a desire to promote his legitimacy by starting from a clean slate. He decided to name his new capital "Rattanakosin" ("Keeping place of the Emerald Buddha"). Rama I also raised various members of his family to royalty. He appointed his brother Surasi (Anuchit Raja) or Maha Sura Singhanat as the "Front Palace" (conventional title of the heir) and his nephew Thong-In or Anurak Devesh as the "Rear Palace".

The King had 42 children. Ten of these were born to Queen Amarinda, the others by various concubines. The Queen's children included Prince Isarasundhorn, later King Buddha Loetla Nabhalai (Rama II) (whom the King appointed as Front Palace after the death of Maha Sura Singhanat in 1803), Prince Maha Senanurak and Prince Maha Sakdi Polsep.

In 1784 - 1785, the first of the Nguyễn Lords, Nguyễn Ánh, convinced Rama I to give him forces to attack Vietnam, which was then under the control of the Tây Sơn brothers. However, the joint Nguyễn - Siam fleet was destroyed in the Battle of Rach Gam – Xoai Mut in the Mekong Delta region. Nguyễn's appeal for Siamese assistance enabled the Siamese to exert considerable political influence over Nguyễn's court. Mac Tu Sinh, the son of Mạc Thiên Tứ and his Siamese wife, was raised among the Siamese, and held office as the governor of Hà Tiên until his death in 1787. Ngo Ma, a general of Siamese descent, was appointed as its acting governor in Mac's place. Nguyễn Ánh also took refuge in Siam at the King's court waiting for the opportunities to defeat Tây Sơn. These episodes demonstrated Rama I's willingness to extend Siamese power beyond his Kingdom.

In Cambodia, King Reamraja of Cambodia was deposed in 1779 and the throne was given to his son, the young Ang Eng. However, the pro-Vietnamese policies of certain Cambodian aristocrats under Ang Eng alarmed Rama I. As a result, Rama I had Ang Eng captured and deported to Bangkok, where he became Rama's adopted son to implant pro-Siamese sentiments on him. Rama I also imposed Chao Phraya Abhaya Bhubet as the Regent of Cambodia.

Nguyễn Ánh secretly left for Vietnam in 1787, leaving Rama I a note. Nguyen managed to recapture Saigon by 1788 and later ascended as Emperor Gia Long in 1802.

In 1794, upon Ang Eng's majority, Rama I reinstalled him as the Narairaja III of Cambodia. The area around Siemreap and Battambang was annexed by Siam, and were governed by Abhaya Bhubet. However, Rama I allowed these territories to be ruled in accordance with Cambodian traditions.

Soon King Bodawpaya of Burma started to pursue his ambitious campaigns to expand his dominions over Siam. The Burmo - Siamese War (1785 – 1786), also known in Siam as the "Nine Armies War" because the Burmese came in nine armies, broke out. The Burmese soldiers poured into Lanna and Northern Siam. Siamese forces, commanded by Kawila, Prince of Lampang, put up a brave fight and delayed the Burmese advance, all the while waiting for reinforcements from Bangkok. When Phitsanulok was captured, Anurak Devesh the Rear Palace, and Rama I himself led Siamese forces to the north. The Siamese relieved Lampang from the Burmese siege.

In the south, Bodawpaya was waiting at Chedi Sam Ong ready to attack. The Front Palace was ordered to lead his troops to the south and counter - attack the Burmese coming to Ranong through Nakhon Si Thammarat. He brought the Burmese to battle near Kanchanaburi. The Burmese also attacked Thalang (Phuket), where the governor had just died. Chan, his wife, and her sister Mook gathered the local people and successfully defended Thalang against the Burmese. Today, Chan and Mook are revered as heroines because of their opposition to the Burmese invasions. In their own lifetimes, Rama I bestowed on them the titles Thao Thep Kasattri and Thao Sri Sunthon.

The Burmese proceeded to capture Songkhla. Upon hearing the news, the governors of Phatthalung fled. However, a monk named Phra Maha encouraged the citizens of the area to take up arms against the Burmese, his campaign was also successful. Phra Maha was later raised to the nobility by Rama I.

As his armies were destroyed, Bodawpaya retreated. The next year, he attacked again, this time constituting his troops as a single army. With this force Bodawpaya passed through the Chedi Sam Ong pass and settled in Ta Din Dang. The Front Palace marched the Siamese forces to face Bodawpaya. The fighting was very short and Bodawpaya was quickly defeated. This short war was called the "Ta Din Dang campaign".

Chinese immigration increased during Rama I's reign, who maintained Taksin's policy of allowing Chinese immigration to sustain the country's economy. The Chinese were found mainly in the trading and mercantile sector, and by the time his son and grandson came to the throne, European explorers noted that Bangkok was filled with Chinese junks of all sizes.

Rama I moved the capital from Thonburi, which was founded by his predecessor Taksin, and built the new capital Bangkok. During the first few years prior to the founding of the current capital, he saw the construction of the palaces and the Chapel Royal. The Chapel Royal or Wat Phra Kaew of which the Emerald Buddha is enshrined is located within his Royal Palace or the Grand Palace. With the completion of the new capital, Rama I held an official ceremony naming the new capital.

In 1804, Rama I began the compilation of the Three Seals Law, consisting of old Ayutthayan laws collected and organized. He also initiated a reform of government and the style of Kingship.

Rama I was also noted for instituting major reforms in Buddhism as well as restoring moral discipline among the monks in the country, which had gradually eroded with the fall of Ayutthaya. Monks had already dabbled in superstitions when he first came to power, and Rama I implemented a law which required a monk who wished to travel to another principality for further education to present a certificate bearing his personal particulars, which would prove a monk own's legitimacy that he had been properly ordained. The King also repeatedly emphasised in state ceremonies to place devotion to the Buddha, and not over guardian spirits and past rulers, of which vestiges of ancient Animist worship had persisted among the Thais prior to his rule.

The King also appointed the first Supreme Patriarch of Thai Buddhism, whose responsibilities included the duty of ensuring that Rama I's laws are maintained which was to ensure law and order within the Buddhist Sangha. Rama I's passion for literature was also connected with his concern for Buddhist order within the country. He was noted for advocating a Thai translation of important Pali works. Buddhist texts lost in the chaos after the sacking of Ayutthaya by the Burmese in 1767 were salvaged under the direction of Rama I. He also wrote a Thai version of the Ramayana epos called Ramakian.

Also, Rama I renewed the relationship with the Vatican and the Jesuits. Missionaries who were expelled during Taksin's reign, were invited back to Siam. Catholic missionaries's activities then continued in Siam. Reportedly the numbers of local catholics increased steadily to thousands as their churches were protected, gaining freedom to propagate their belief again.

King Rama I died on 7 September 1809 after a short but acute illness. He was succeeded by his son Prince Isarasundhorn as Buddha Loetla Nabhalai or Rama II.

Siam during the reign of Rama I reached a new height of power not seen since the sixteenth century. Militarily Siam was able to successfully repel Burmese invasions and exerted influences over Laos and Cambodia and even Vietnam. Culturally Rama I also encouraged cultural works to rehabilitate people after the successive series of wars and built many temples and monuments during his reign. His policies laid the foundation for Siam to expand within the next decades.

The 6th of April is Chakri Memorial Day, a holiday to commemorate the founder of the Chakri Dynasty.