Phrabat Somdet Phra Boromaracha Thirat II () or Chao Sam Phraya () (- 1448) was the king of Ayutthaya kingdom. His reign saw early expansions of Ayutthaya.
Chao Sam Phraya (lit. The Third Chao Phraya) was a son of Nagarindra Thirat who had finally taken the Ayutthayan throne for the Supannabhum dynasty. Chao Sam Phraya had two elder brothers – Chao Ai Phraya (lit. The First Chao Phraya) and Chao Yi Phraya (lit. The Second Chao Phraya). Chao Sam Phraya was appointed by his father to govern Chainat. In 1424, Nagarindra Thirat died. The two brothers – Chao Ai Phraya and Chao Yi Phraya – from their respective cities marched to Ayutthaya for the throne. They fought and both died, leaving the throne to Chao Sam Phraya.

In 1431, Chao Sam Phraya led Siamese forces to subjugate Cambodia plundering Angkor Thom. This assault eventually caused the Khmer to abandon Angkor and relocate their capital further south. The conquest, however, brought in a large influx of Khmer culture and traditions into the Siamese court. For example, the high reverence of Thai kingship as a deity – known as Devaraja – came from Cambodia.

Chao Sam Phraya also sought northward expansion. He married a queen from Sukhothai and produced a son – Prince Ramesuan. When the last king of Sukhothai died in 1446, the throne was inherited by Prince Ramesuan – further strengthening Ayutthayan control over Sukhothai.

In 1442, Chao Sam Phraya led his armies to conquer Lanna. He managed to sack Chiang Mai but unable to put the death on the Lanna kingdom.

Chao Sam Phraya died in 1448 – to be succeeded by his son Prince Ramesuan as Trilokanat.
Category:1448 deathsCategory:Thai monarchsCategory:Rulers of Ayutthayade:Boromaracha II.ja:サームプラヤーth:สมเด็จพระบรมราชาธิราชที่ 2