Thomas the Apostle (Greek: Θωμᾶς, romanized: Thōmâs; Classical Syriac: ܬܐܘܡܐ, romanized: Tʾōmā, meaning 'the Twin'), also known as Didymus (Greek: Δίδυμος, romanized: Dídymos 'twin'), was one of the Twelve Apostles of Jesus according to the New Testament. Thomas is commonly known as "doubting Thomas" because he initially doubted the resurrection of Jesus when he was told of it (as is related in the Gospel of John); he later confessed his faith ("The lord of me and the God of me") on seeing the places where the wounds appeared still fresh on the holy body of Jesus after the Crucifixion of Jesus.

According to traditional accounts of the Saint Thomas Christians of Kerala, Thomas travelled outside the Roman Empire to preach the Gospel, traveling through southern India in the modern states of Kerala and Tamil Nadu, and eventually reached Muziris (modern-day North Paravur and Kodungalloor) in 52 AD. He started the Church of the East in the region around the Van Province, Edessa, and Hakkari regions of Upper Mesopotamia (modern-day southeastern Turkey), presumably sometime between mid-late 30s and 45, and spent many years evangelizing both Jews and pagans in the area before heading further east in the late 40s. In 1258, some of the relics were brought to Ortona, in Abruzzo, Italy, where they have been held in the Church of Saint Thomas the Apostle. He is regarded as the patron saint of India among its Christian adherents, and the Feast of Saint Thomas on July 3 is celebrated as Indian Christians' Day. The name Thomas remains quite popular among the Saint Thomas Christians of the Indian subcontinent.

Many churches in the Middle East and southern Asia, besides India, also mention Apostle Thomas in their historical traditions as being the first evangelist to establish those churches, the Church of the East, as well as the early church of Sri Lanka.

Thomas first speaks in the Gospel of John. In John 11:16, when Lazarus has recently died, and the apostles do not wish to go back to Judea, Thomas says: "Let us also go, that we may die with him."

Thomas speaks again in John 14:5. There, Jesus had just explained that he was going away to prepare a heavenly home for his followers, and that one day they would join him there. Thomas reacted by saying, "Lord, we know not whither thou goest; and how can we know the way?"

John 20:24–29 tells how doubting Thomas was skeptical at first when he heard that Jesus had risen from the dead and appeared to the other apostles, saying, "Except I shall see on his hands the print of the nails, and put my finger into the print of the nails, and thrust my hand into his side, I will not believe." But when Jesus appeared later and invited Thomas to touch his wounds and behold him, Thomas showed his belief by saying, "My Lord and my God". Jesus then said, "Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed."

The name Thomas (Greek: Θωμᾶς) given for the apostle in the New Testament is derived from the Aramaic תְּאוֹמָא Tʾōmā (Syriac ܬܐܘܿܡܵܐ/ܬ݁ܳܐܘܡܰܐ Tʾōmā/Tāʾwma), meaning "the twin" and cognate to Hebrew תְּאוֹם tʾóm. The equivalent term for twin in Greek, which is also used in the New Testament, is Δίδυμος Didymos.

The Nag Hammadi copy of the Gospel of Thomas begins: "These are the secret sayings that the living Jesus spoke and Didymus, Judas Thomas, recorded." Early Syrian traditions also relate the apostle's full name as Judas Thomas. Some have seen in the Acts of Thomas (written in east Syria in the early 3rd century, or perhaps as early as the first half of the 2nd century) an identification of Thomas with the apostle Judas, Son of James. However, the first sentence of the Acts follows the Gospels and the Acts of the Apostles in distinguishing the apostle Thomas and the apostle Judas son of James. Others, such as James Tabor, identify him as Jude, brother of Jesus mentioned by Mark. In the Book of Thomas the Contender, part of the Nag Hammadi library, he is alleged to be a twin to Jesus: "Now, since it has been said that you are my twin and true companion, examine yourself…"