Mahathammarachathirat (Thai: มหาธรรมราชาธิราช) or Sanphet I (Thai: สรรเพชญ์ที่ 1), or Maha Tammaraja, formerly known as Khun Phirenthrathep (Thai: ขุนพิเรนทรเทพ), or Khun Piren, was the first King of Ayutthaya Kingdom of the Sukhothai dynasty ruling from 1569 to 1590. As a powerful Sukhothai noble, Phirenthrathep gradually rose to power. After playing many political turns, he was eventually crowned as the King of Siam.
Mahathammarachathirat died in 1590. He was succeeded by Naresuan.