Sanpet III (Thai: สรรเพชญ์ที่ 3, Khmer: សព៌េជ្ញទី៣) or Ekathotsarot (Thai: เอกาทศรถ, Khmer: ឯកាទឝរថ or ឯកាទសរថ;-1620) was the King of Ayutthaya from 1605 to 1620 and overlord of Lan Na 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.

The White Prince was the son of Maha Thammarachathirat of Phitsanulok and Queen Wisutkasat. White Prince had an elder brother who was epitheted The Black Prince and an elder sister the Golden Princess.

In November 1563, Phitsanulok came under attack by King Bayinnaung of Burma. Faced with an overwhelming force, Maha Thammarachathirat surrendered in January 1564, and agreed to join Bayinnaung's assault on Ayutthaya. With Phitsanulok's help, Bayinnaung forced King Maha Chakkraphat of Ayutthaya to surrender in February 1564. Bayinnaung brought back the Black Prince and White Prince, along with Ayutthaya king Maha Chakkraphat. The two princes were educated and overseen by Bayinnaung along with other captive princes.

When Ayutthaya revolted in May 1568, Maha Thammarachathirat remained loyal to Bayinnaung, and became the vassal king of Siam when Bayinnaung's forces retook Ayutthaya in August 1569. The Black and White Prince then returned to Ayutthaya in 1571.

Prince Ekatotsarot joined his brother Naresuan in various wars with the Burmese. Naresuan declared independence in May 1584, and fought off a series of Burmese invasions from 1584 to 1593. In 1590, Maha Thammarachathirat died. Naresuan was crowned as the King of Ayutthaya while Ekatotsarot was made Uparaja but with equal honor to Naresuan (As in the case of Mongkut and Pinklao).

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