Hattusili III (Hittite: "from Hattusa") was a king of the Hittite empire (New Kingdom or Late Empire) ca. 1267–1237 BC (short chronology). He was the fourth and last son of Mursili II. Mursili appointed Hattusili as priest of Sausga of Samuha, and Hattusili remained loyal to the "Ishtar of Samuha" to the end of his days.
His older brother Muwatalli II moved his seat in Tarhuntassa and appointed him governor of Hattusa. Hattusili was a commander of Hittite forces during the famous Battle of Kadesh against Egypt in 1274 BC. Hattusili as governor, also reconquered Nerik and became the High Priest of its storm god. Hattusili named his eldest son and crown prince "Nerikkaili" in honor of this achievement. Hattusili married priestess of Ishtar, Puduhepa who later became Hittite queen and Tawanannas.
His nephew, Mursili III (or Urhi-Teshub) moved the capital back to Hattusa (KBo 21.15 i 11-12), rendering Hattusili's governorship redundant. Mursili then deposed him from Nerik, triggering a civil war. Hattusili defeated and exiled his nephew. Hattusili renamed Urhi-Teshup's sibling Ulmi-Teshup "Kurunta" (or: married her off to Kurunta), and appointed Kurunta over Tarhuntassa in his place. After this, Hattusili elevated a junior son Tudhaliya IV as crown prince instead.
Hattusili and the Egyptian pharaoh Ramesses II sealed a written peace treaty based upon Hittite models which, due to Egyptian monumental copies of it, has become the earliest well known treaty in history, establishing a long-lasting peace between the two rival empires. Ramesses married Hattusili's daughter, known the by the Egyptian name of Maathorneferure. Years later he married another Hittite princess.
An archive of over 200 letters have been found from the royal palace at Hattusa which show that Hattusili exchanged letters with numerous Near Eastern kings including Ramesses II of ancient Egypt. They are an important primary source for this period.