Adonizedek (variously transliterated as Adoni-zedec or Adoni-Zedek (in Hebrew, Adoni-Tzedek) was, according to the Book of Joshua, king of Jerusalem at the time of the Israelite invasion of Canaan (Joshua 10:1-3). His name means "my lord is righteousness" in Hebrew.
Adonizedek led a coalition of five of the neighboring Amorite rulers (Hoham king of Hebron, and unto Piram king of Jarmuth, and unto Japhia king of Lachish, and unto Debir king of Eglon) to resist the invasion, but the allies were defeated at Gibeon, and suffered at Beth-horon, not only from their pursuers, but also from a great hail-storm. The five allied kings took refuge in a cave at Makkedah and were imprisoned there until after the battle, when Joshua commanded that they be brought before him; whereupon they were brought out, humiliated, and put to death.
According to the Midrash, the name Adoni-zedek is translated as "Master of Zedek" — that is, "of Jerusalem," the city of righteousness (Genesis Rabbah xliii. 6).
The author of the article for the Easton's Bible Dictionary states that amongst the Amarna letters are some letters from Adonizedek to the Pharaoh of Egypt, which add to the history recorded in Josh. 10. However, the only king of Jerusalem mentioned in this archive is one `Abdi-Heba (whose name translates as "servant of Heba"), who is said to have succeeded Lab'ayu. Six of his letters to the king of Egypt (EA 285-290) are included in the Amarna letters, and he is mentioned in a seventh (EA 280). Perhaps the Dictionary author saw how `Abdi-Heba complained of the raids by the Habiru, who at the time were unquestioningly identified with the Hebrews, and forced the identification.