John the Baptist (c. 1st century BC – c. AD 30) was a Judaean preacher active in the area of the Jordan River in the early 1st century AD. He is also known as John the Forerunner in Christianity, John the Immerser in some Baptist Christian traditions, and Prophet Yahya in Islam. He is sometimes alternatively referred to as John the Baptiser.

John is mentioned by the Roman Jewish historian Josephus and he is revered as a major religious figure in Christianity, Islam, the Baháʼí Faith, the Druze Faith, and Mandaeism, the latter in which he is considered to be the final and most vital prophet. He is considered to be a prophet of God by all of the aforementioned faiths, and is honoured as a saint in many Christian denominations. According to the New Testament, John anticipated a messianic figure greater than himself, and the Gospels portray John as the precursor or forerunner of Jesus. Jesus himself identifies John as "Elijah who is to come", which is a direct reference to the Book of Malachi (Malachi 4:5), that has been confirmed by the angel who announced John's birth to his father, Zechariah. According to the Gospel of Luke, John and Jesus were relatives.

Some scholars maintain that John belonged to the Essenes, a semi-ascetic Jewish sect who expected a messiah and practiced ritual baptism. John used baptism as the central symbol or sacrament of his pre-messianic movement. Most biblical scholars agree that John baptized Jesus, and several New Testament accounts report that some of Jesus' early followers had previously been followers of John.

According to the New Testament, John was sentenced to death and subsequently beheaded by Herod Antipas around AD 30 after John rebuked him for divorcing his wife Phasaelis and then unlawfully wedding Herodias, the wife of his brother Herod Philip I. Josephus also mentions John in the Antiquities of the Jews and states that he was executed by order of Herod Antipas in the fortress at Machaerus.

Followers of John existed well into the 2nd century AD, and some proclaimed him to be the messiah. In modern times, the followers of John the Baptist are the Mandaeans, an ancient ethnoreligious group who believe that he is their greatest and final prophet.