The beheading of John the Baptist, also known as the decollation of Saint John the Baptist or the beheading of the Forerunner, is a biblical event commemorated as a holy day by various Christian churches. According to the New Testament, Herod Antipas, ruler of Galilee under the Roman Empire, had imprisoned John the Baptist because he had publicly reproved Herod for divorcing his first wife and unlawfully taking his sister-in-law (his brother's wife) Herodias as his second wife. He then ordered him to be killed by beheading.

As a non-Biblical source, Jewish historian Josephus also recounts that Herod had John imprisoned and killed due to "the great influence John had over the people", which might persuade John "to raise a rebellion". Josephus also writes that many of the Jews believed that Herod's later military disaster was God's punishment for his treatment of John.

According to the synoptic Gospels, Herod, who was tetrarch, or sub-king, of Galilee under the Roman Empire, had imprisoned John the Baptist because he reproved Herod for divorcing his wife (Phasaelis, daughter of King Aretas of Nabataea) and unlawfully taking Herodias, the wife of his brother Herod Philip I. On Herod's birthday, Herodias' daughter (whom Josephus identifies as Salome) danced before the king and his guests. Her dancing pleased Herod so much that in his drunkenness he promised to give her anything she desired, up to half of his kingdom. When Salome asked her mother what she should request, she was told to ask for the head of John the Baptist on a platter. Although Herod was appalled by the request, he reluctantly agreed and had John executed by beheading in the prison. In art, the episode is known as The Feast of Herod.

Jewish historian Josephus also relates in his Antiquities of the Jews that Herod killed John, stating that he did so, "lest the great influence John had over the people might put it into his [John's] power and inclination to raise a rebellion, (for they seemed ready to do any thing he should advise), [so Herod] thought it best [to put] him to death." He further states that many of the Jews believed that the military disaster that fell upon Herod at the hands of Aretas, his father-in-law (Phasaelis' father), was God's punishment for his unrighteous behavior.

None of the sources gives an exact date, which was probably in the years 28–29 AD (Matthew 14:1–12; Mark 6:14–27; Luke 9:9) after imprisoning John the Baptist in 27 AD (Matthew 4:12; Mark 1:14) at the behest of Herodias, his brother's wife, whom he took as his mistress (Matthew 14:3–5; Mark 6:17–20). According to Josephus, the death took place at the fortress of Machaerus.

The following comparison table is primarily based on the New International Version (NIV) English translation of the New Testament. The account of Flavius Josephus in Antiquities of the Jews was translated by William Whiston.

The liturgical commemoration of the Beheading of St. John the Baptist is almost as old as that commemorating his birth, which is one of the oldest feasts, if not the oldest, introduced into both the Eastern and Western liturgies to honour a saint.

The Roman Catholic Church celebrates the feast on 29 August, as does the Lutheran Church. Many other churches of the Anglican Communion do so as well, including the Church of England, though some designate it a commemoration rather than a feast day.