Onkelos is the name of a famous convert to Judaism in Talmudic times. He is considered to be the author of the famous Targum Onkelos.

Onkelos is mentioned several times in the Talmud. According to the traditional Jewish sources, he was a prominent Roman nobleman, a nephew of the Roman emperor Titus. His conversion is the subject of a story whereupon he first consulted with the spirits of three deceased enemies of Israel to see how Israel fared in the next world (Gittin 56b). The first was his uncle Titus, who was blamed for the destruction of the Second Temple; the second was the seer Balaam, hired by Balak king of Moab to curse Israel; and the last was Yeshu, a name used for those who sought to lead Jews astray to idolatry, in particular an idolatrous former student of Rabbi Joshua ben Perachiah in the Hasmonean period. Onkelos is said to have seen all of them subject to humiliating punishments for harming Israel. All three urged Onkelos to complete his conversion. (Some sources give the subject of these stories as Aquila, a convert to Judaism who translated the Bible in Greek. The stories may then have been transferred to Onkelos, the similar names being confused.)

After his conversion, the Talmud records a story of how the Roman emperor tried to have Onkelos arrested (Avodah Zarah 11a). Onkelos cited verses from the Tanakh to the first Roman legion, who then converted. The second legion was also converted, after he juxtaposed God's personal guidance of Israel in the Book of Numbers to the Roman social hierarchy. A similar tactic was used for the third legion, where Onkelos compared his mezuzah to a symbol of God guarding the home of every Jew, in contrast to a Roman king who has his servants guard him. The third legion also converted and no more were sent.

According to tradition, Onkelos authored his Targum as an exposition of the "official" interpretation of the pshat (or basic meaning) of the Torah, as received by Rabbi Eliezer . This helped canonise the status of both Onkelos and his Targum in the Jewish tradition.