Onesimus (Ancient Greek: Ὀνήσιμος, romanized: Onēsimos, meaning "useful") was a Christian mentioned in the New Testament. He was a slave to Philemon, a Christian, and is the subject of Paul's Epistle to Philemon.

He may also be the same Onesimus mentioned by Ignatius of Antioch (died c. 107) as bishop in Ephesus. Eastern Orthodox tradition also list an Onesimus as the third bishop of Byzantium.

The name "Onesimus" appears in two of Paul's epistles. The Epistle to Philemon was written by Paul the Apostle to Philemon concerning a runaway slave named Onesimus. Onesimus turned up where Paul was imprisoned (Rome or Caesarea Maritima) to escape punishment for a theft of which he was accused. After hearing the Gospel from Paul, Onesimus converted to Christianity. Paul, having earlier converted Philemon to Christianity, sought to reconcile the two by writing the letter to Philemon which today exists in the New Testament. The letter reads (in 1:10-16):

I appeal to you for my son Onesimus, whom I have begotten while in my chains, who once was unprofitable to you, but now is profitable to you and to me. I am sending him back. You therefore receive him, that is, my own heart, whom I wished to keep with me, that on your behalf he might minister to me in my chains for the gospel. But without your consent I wanted to do nothing, that your good deed might not be by compulsion, as it were, but voluntary. For perhaps he departed for a while for this purpose, that you might receive him forever, no longer as a slave but more than a slave — a beloved brother, especially to me but how much more to you, both in the flesh and in the Lord.

In the Epistle to the Colossians 4:9 a person of this name is identified as a Christian accompanying Tychicus to visit the Christians in Colossae; nothing else is stated about him in this context. He may well be the freed Onesimus from the Epistle to Philemon.

He may also be the same Onesimus named by Ignatius of Antioch (died c. 107) as bishop in Ephesus In his Epistle written to the Ephesians while on his way to be executed in Rome, Ignatius wrote:

I received, therefore, your whole multitude in the name of God, through Onesimus, whose love surpasses words, in the flesh as your bishop. I pray that you may love him with a love according to Jesus Christ and that you may all be like him. For blessed is He Who granted unto you, worthy as you are, to possess such a bishop.

Onesimus must have accepted episcopal see of Ephesus following Saint Timothy. If so, Onesimus went from slave to brother to bishop. According to Tradition), Onesimus was imprisoned and may have been martyred by stoning (some sources claim he was beheaded). Given that Ignatius of Antioch died under Emperor Trajan (97-117), Onesimus's death more likely fell under that Emperor as well.