Chairachathirat (Thai: ไชยราชาธิราช), or Chai reigned 1534–1546 as King of the Ayutthaya kingdom of Siam. His reign was remarkable for the influx of Portuguese traders, mercenaries, and early Modern warfare technology.

Prince Chairachathirat was a son of King Ramathibodi II. In 1533, following the death of his brother Borommaracha IV, his nephew Prince Ratsadathirat (Borommaracha IV's son) succeeded the Ayutthayan throne. Chairachathirat was then appointed the Uparaja of Pitsanulok.

Government authority under five-year-old Ratsadathirat proved to be weak. In 1534, only five months after his nephew's ascension, Chairacha marched to Ayutthaya to stage a coup, killed his nephew, and took the throne of Ayutthaya.

In 1539, King Tabinshwehti attacked the Mon people inhabiting Chiang Kran, but under Siamese rule. Chairachathirat "marched against Chiang Kran" with the aid of 120 Portuguese traders in Ayutthaya, and drove the Burmese back. The Portuguese were then allowed to build a church near the Takhian Canal to practice their religion.

Chairacha appointed his brother Prince Tianracha (later Maha Chakkrapat) as the Uparaja but did not granted him the title of King of Sukhothai as Chairacha tried to unite the two kingdoms by reducing the power of Sukhothai nobles. He also called the Sukhothai nobles to Ayutthaya to move them from their base at Pitsanulok and made Ayutthaya the sole center of authority.

Though not having a son by a queen, Chai did have one, Yodfa, from Lady Sudachan, his favorite Chao Chom. However, she had a secret lover, Worawongsathirat, an officer of the Guards. Returning from the north, King P'ra Chai became ill and died, and suspicion fell on Lady Sudachan. Her eleven-year-old son, Yodfa, became king in 1546. Chai's half brother, Prince Tien became regent, but entered a monastery to escape the ensuing power struggle. Sudachan became pregnant by Worawongsathirat and Yodfa was soon murdered. Khun Worawongsa assumed power. Khun Piren, and other retired officers, then ambushed the king and queen, and placed Prince Tien on the throne in 1549.

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