The New Covenant (Ancient Greek: διαθήκη καινή, romanized: diathḗkē kainḗ) is a biblical interpretation which was originally derived from a phrase in the Book of Jeremiah (Jeremiah 31:31–34), in the Hebrew Bible (or the Old Testament of the Christian Bible). Generally, Christians believe that the promised New Covenant—new relationship with God—was instituted at the Last Supper as part of the Eucharist, which, in the Gospel of John, includes the New Commandment.

Most Christians believe that Jesus is the mediator of the New Covenant, and they also believe that the blood of Christ, which was shed during his crucifixion, is the only blood sacrifice which is required by the covenant. Based on the biblical passage in the Epistle to the Hebrews (9:16–17) which reads that, "Where a will is involved, the death of the one who made it must be established. For a will takes effect only at death, since it is not in force as long as the one who made it is alive." Protestants tend to believe that the New Covenant came into force with the death of Jesus the Christ, and the commentary to the Roman Catholic New American Bible also says that Christ is the "testator whose death puts his will into effect".

There are several Christian eschatologies that further define the New Covenant. For example, an inaugurated eschatology defines and describes the New Covenant as an ongoing relationship between Christian believers and God that will be in full fruition after the Second Coming of Christ; that is, it will not only be in full fruition in believing hearts, it will also be in fruition in the world to come. The description of the connection between the blood of Christ and the New Covenant is contained in most modern English translations of the New Testament such as the Luke 22:20 which reads: "this cup that is poured out for you is the new covenant in my blood".

The key New Testament chapter for the Christian concept of the New Covenant is Hebrews 8, a portion of which is quoted below:

But now Jesus has obtained a superior ministry, since the covenant that he mediates is also better and is enacted on better promises. For if that first covenant had been faultless, there would have been no occasion to look for a second. For he finds fault with them when he says: "Behold, the days are coming, declares the Lord, when I will establish a new covenant with the house of Israel and with the house of Judah, not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt. For they did not continue in my covenant, and so I showed no concern for them, declares the Lord. For this is the covenant that I will make with the house of Israel after those days, declares the Lord: I will put my laws into their minds, and write them on their hearts, and I will be their God, and they shall be my people. And they shall not teach, each one his neighbor and each one his brother, saying, 'Know the Lord,' for they shall all know me, from the least of them to the greatest. For I will be merciful toward their iniquities, and I will remember their sins no more." In speaking of a new covenant, he makes the first one obsolete. And what is becoming obsolete and growing old is ready to vanish away.

— Hebrews 8:6–13

That full quotation, with partial quotations of the same text in other New Testament passages, reflects that the authors of the New Testament and Christian leaders generally, consider Jeremiah 31:31–34 to be a central Old Testament prophecy of the New Covenant. Here is the key text:

Behold, the days are coming, declares the Lord, when I will make a new covenant with the house of Israel and the house of Judah, not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt, my covenant that they broke, though I was their husband, declares the Lord. For this is the covenant that I will make with the house of Israel after those days, declares the Lord: I will put my law within them, and I will write it on their hearts. And I will be their God, and they shall be my people. And no longer shall each one teach his neighbor and each his brother, saying, "Know the Lord," for they shall all know me, from the least of them to the greatest, declares the Lord. For I will forgive their iniquity, and I will remember their sin no more.