Usermaatre Setepenamun Osorkon III Si-Ese is Pharaoh of Egypt in the 8th century BCE.

He is the same person as the Crown Prince and High Priest of Amun Osorkon B, son of Takelot II by his Great Royal Wife Karomama II.

Prince Osorkon B is best attested by his Chronicle—which consists of a series of texts documenting his activities at Thebes—on the Bubastite Portal at Karnak.

He later reigns as king Osorkon III in Upper Egypt for twenty-eight years after defeating the rival forces of Pedubast I/Shoshenq VI who had apparently resisted the authority of his father here.

Osorkon rules the last five years of his reign in coregency with his son, Takelot III, according to Karnak Nile Level Text No.

13.

Osorkon III's formal titulary is long and elaborate: Usermaatre Setepenamun, Osorkon Si-Ese Meryamun, Netjer-Heqa-waset.

Osorkon III's precise accession date is unknown.

Various Egyptologists have suggested it may have been from around the mid-790s BCE to as late as 787 BCE.

The issue is complicated by the fact that Prince Osorkon B did not immediately declare himself king after his successful conquest of Thebes and defeat of Shoshenq VI.

This is evidenced by the fact that he dated this seminal event to Year 39 of Shoshenq III rather than Year 1 of his reign.

Osorkon III may, therefore, have waited for a minimum of one or two years before proclaiming himself as a Pharaoh of the Theban-based 23rd Dynasty.