Answer
The Prayer of Manasseh is a part of the Apocrypha. It is a short work, containing just 15 verses. It purports to be a prayer by King Manasseh of Judah (697-642 B.C.), but it was pseudonymously written as early as the second century or just before the destruction of Jerusalem in A.D. 70. Second Chronicles 33:19 says that Manasseh prayed but does not record the prayer itself referring the reader to what was “written in the records of the seers.”
King Manasseh, the thirteenth king of Judah, was one of the most wicked and idolatrous kings in biblical history (2 Kings 21:1-18). He was captured by the Assyrians and imprisoned in Babylon. There, he prayed for mercy and repented of his sin of idolatry (2 Chronicles 33:1-19).
The Prayer of Manasseh is considered by Jews, Catholics and Protestants as apocryphal, i.e., non-canonical and of doubtful authenticity. However, the fourth-century Vulgate included it at the end of the book of 2 Chronicles. It later became part of the Matthew Bible and the Geneva Bible of 1599. It is also found in the Apocrypha of the King James Bible.
The prayer departs from Christian teaching in that it says men such as Abraham, Isaac, and Jacob did not need to repent because they “did not sin” (verse 8). This runs counter to the clear teaching of Scripture that all have sinned (Romans 3:10-12; Romans 3:21-26). The righteousness of Abraham was a product of his faith in God and was not anything inherent in him (Romans 4:3; Philippians 3:8-9).
In summary, God has told us that Manasseh prayed a much-needed prayer of repentance, but He has not told us the content of that prayer.
