Sanpet III (Thai: สรรเพชญ์ที่ 3, Khmer: សព៌េជ្ញទី៣) or Ekathotsarot (Thai: เอกาทศรถ, Khmer: ឯកាទឝរថ or ឯកាទសរថ) was the King of Ayutthaya from 1605 to 1620 and Emperor of Lan Na Kingdom from 1605 to 1608/09 succeeding his brother Naresuan. His reign was mostly peaceful as Siam was then a powerful state through the conquests of Naresuan. Also during his reign that foreigners of various origin began to fill the mercenary corps. In particular, the king had a regiment of professional Japanese guards under the command of Yamada Nagamasa.
During Ekathotsarot's reign, the English first came to Siam in 1612.