Sanphet IX (Thai: สรรเพชญที่ ๙) or Thai Sa (Thai: ท้ายสระ) was the King of Ayutthaya from 1709 to 1732 and the third ruler of the Ban Phlu Luang Dynasty.
King Thai Sa had three sons - Prince Naren, Prince Abhai and Prince Paramet. In 1732, while King Thai Sa was on his deathbed he gave the throne to Prince Abhai since Prince Naren, his elder brother had entered the priesthood. The two remaining princes rose in armed rebellion against their uncle. Prince Phon, the younger son of Suriyenthrathibodi managed to defeat his nephews and took the throne as King Borommakot.