Melchizedeck is the first priest ("the priest of the most high God") mentioned in the Old Testament. The story of the encounter between Melchizedeck and Abraham (then called Abram) is in chapter 14 of the book of Genesis. This is long before the Hebrew priesthood is established by Moses. Melchizedeck is mentioned one more time in the Old Testament in the book of Psalms 110:4.
The Lord hath sworn, and will not repent, thou art a priest forever after the order of Melchizedeck.
Psalm 110 is considered a Messianic psalm, which means that it foretells the coming of the Messiah, or Jesus. This verse says that when the Messiah comes, he will be considered a priest just as Melchizedeck was a priest.
Melchizedeck is referred to as the king of Salem, which means king of peace. Additionally, the word Melchizedeck means "king of righteousness."
In Genesis 14, an alliance of kings attacks the city of Sodom and some other nearby cities. At the time, Abram's nephew, Lot, is living in Sodom. When the city falls, Lot is taken captive. After Abram hears of it, he arms his servants and goes out in pursuit of the marauders. He defeats the raiding kings, recovers Lot and all the goods of the king of Sodom, and returns. It is when the king of Sodom meets triumphant Abram that Melchizedeck appears.
Melchizedeck gives Abram a blessing, and Abram gives Melchizedeck tithes (an offering of 10 percent) of all the spoils. The king of Sodom offers Abram a reward, but Abram refuses to accept it.
This is the only place in the Bible in which Melchizedeck plays an active role. As mentioned above, he is referred to in Psalm 110. He is also mentioned in the book of Hebrews chapter 7 verses 1 to 4 in the New Testament, in which Genesis chapter 14 is retold by the apostle Paul.