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.

Though the Kingdom of Sukhothai had come under personal union with Ayutthaya since 1448, the royal clan of Sukhothai still held power in their base Phitsanulok and constitutes as one of four political clans of 16th century Ayutthaya (Supannabhum, Uthong, Sukhothai, and Sri Thamnakorn). Chairacha, however, tried to reduce the power of Sukhothai nobles. He ceased to appoint the Uparaja the King of Sukhothai and called the Sukhothai nobles to Ayutthaya to dissolve their base of power at Phitsanulok.

Khun Phirenthrathep was one of the Sukhothai nobles at the court of Ayutthaya. In 1548, the kingdom fell under the governance of Worawongsathirat and Si Suda Chan of the Uthong clan. The Uthong clan rose to power at the expense of other clans. Khun Pirenthrathep then sought alliance with Sri Thamnakorn clan led by Khun Inthrawongse and staged a coup against Worawongsathirat and Si Suda Chan in 1548, restoring the throne to Supannabhum dynasty.

Maha Chakkraphat made Khun Phirenthrathep (who had staged a coup and put him on the throne) as Mahathammarachathirat. The name Mahathammaracha was the reigning name of Sukhothai kings in the 14th century. Mahathammarachathirat enjoyed a great power. He married Maha Chakkraphat's daughter, Sawatdirat, as his queen with the name Queen Wisutkasat.

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