Phra Phutthayotfa Chulalok Maharaj (Thai: พระพุทธยอดฟ้าจุฬาโลกมหาราช, 20 March 1737 – 7 September 1809), personal name Thongduang (ทองด้วง), also known as Rama I, was the founder of the Rattanakosin Kingdom and the first monarch of the reigning Chakri dynasty of Siam (now Thailand). His full title in Thai is Phra Bat Somdet Phra Paramoruracha Mahachakkriborommanat Phra Phutthayotfa Chulalok (พระบาทสมเด็จพระปรโมรุราชามหาจักรีบรมนารถ พระพุทธยอดฟ้าจุฬาโลก). He ascended the throne in 1782, following the deposition of 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 from a Mon male line descent family, great grandson of Kosa Pan. His father served in the royal court of the Ayutthaya Kingdom, 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. Thongduang was the first Somdet Chao Phraya, the highest rank the nobility could attain, equaled to that of royalty. 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 1785–86, which was the last major Burmese assault on Siam.