The deuterocanonical books (from the Greek meaning "belonging to the second canon") are books and passages considered by the Catholic Church, the Eastern Orthodox Church, the Oriental Orthodox Churches, and the Assyrian Church of the East to be canonical books of the Old Testament, but which Protestant denominations regard as apocrypha. They date from 300 BC to 100 AD, mostly from 200 BC to 70 AD, before the definite separation of the Christian church from Judaism. While the New Testament never directly quotes from or names these books, the apostles most frequently used and quoted the Septuagint, which includes them. Some say there is a correspondence of thought, and others see texts from these books being paraphrased, referred, or alluded to many times in the New Testament, depending in large measure on what is counted as a reference.

Although there is no scholarly consensus as to when the Hebrew Bible canon was fixed, some scholars hold that the Hebrew canon was established well before the 1st century AD – even as early as the 4th century BC, or by the Hasmonean dynasty (140–40 BC).

The Septuagint translation of the Hebrew Bible into Greek, which the early Christian church used as its Old Testament, included all of the deuterocanonical books. The term distinguished these books from both the protocanonical books (the books of the Hebrew canon) and the biblical apocrypha (books of Jewish origin that were sometimes read in Christian churches as scripture but which were not regarded as canonical).

The Council of Rome (382 AD) defined a list of books of scripture as canonical. It included most of the deuterocanonical books.