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.
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.