John the Apostle (Ancient Greek: Ἰωάννης; Latin: Ioannes; c. 6 AD – c. 100 AD), also known as Saint John the Beloved and, in Eastern Orthodox Christianity, Saint John the Theologian, was one of the Twelve Apostles of Jesus according to the New Testament. Generally listed as the youngest apostle, he was the son of Zebedee and Salome. His brother James was another of the Twelve Apostles. The Church Fathers identify him as John the Evangelist, John of Patmos, John the Elder, and the Beloved Disciple, and claim that he outlived the remaining apostles and was the only one to die of natural causes, although modern scholars are divided on the veracity of these claims.

John the Apostle is traditionally held to be the author of the Gospel of John, and many Christian denominations believe that he authored several other books of the New Testament (the three Johannine epistles and the Book of Revelation, together with the Gospel of John, are called the Johannine works), depending on whether he is distinguished from, or identified with, John the Evangelist, John the Elder, and John of Patmos.

Although the authorship of the Johannine works has traditionally been attributed to John the Apostle, only a minority of contemporary scholars believe he wrote the gospel, and most conclude that he wrote none of them. Regardless of whether or not John the Apostle wrote any of the Johannine works, most scholars agree that all three epistles were written by the same author and that the epistles did not have the same author as the Book of Revelation, although there is widespread disagreement among scholars as to whether the author of the epistles was different from that of the gospel.

John the Apostle was born into a family of Jewish fishermen on the Sea of Galilee. He was the son of Zebedee and the younger brother of James the Great. According to church tradition, their mother was Salome. Also according to some traditions, Salome was the sister of Mary, Jesus' mother, making Salome Jesus' aunt, and her sons John the Apostle and James were Jesus' cousins.

John the Apostle is traditionally believed to be one of two disciples (the other being Andrew) recounted in John 1:35–39, who upon hearing the Baptist point out Jesus as the "Lamb of God", followed Jesus and spent the day with him, thus becoming the first two disciples called by Jesus. On this basis some traditions believe that John was first a disciple of John the Baptist, even though he is not named in this episode.

According to the Synoptic Gospels (Matt 4:18–22; Mark 1:16–20; Luke 5:1–11), Zebedee and his sons fished in the Sea of Galilee. Jesus then called Peter, Andrew and the two sons of Zebedee to follow him. James and John are listed among the Twelve Apostles. Jesus referred to the pair as "Boanerges" (translated "sons of thunder"). A Gospel story relates how the brothers wanted to call down heavenly fire on an unhospitable Samaritan town, but Jesus rebuked them. John was also the disciple who reported to Jesus that they had 'forbidden' a non-disciple from casting out demons in Jesus' name, prompting Jesus to state that 'he who is not against us is on our side'.

John is traditionally believed to have lived on for more than fifty years after the martyrdom of his brother James, who became the first Apostle to die a martyr's death in AD 44.

John is always mentioned in the group of the first four apostles in the Gospels and in the Book of Acts, listed either second, third or fourth.