Saint Justus (d. 10 November 627), by birth a Roman, was one of the missionaries who was sent to England, by Pope Gregory II, at the request of St. Augustine of Canterbury in 601.

He was appointed to be the first Bishop of Rochester. When persecution broke out after the death of King Aethelbert of Kent, he fled to Gaul; but, a year later, he was reinstated in his bishopric, which he governed with diligence and care until, in 624, he became Archbishop of Canterbury, receiving the pallium from Pope Boniface V. He consecrated Romanus as his successor as Bishop of Rochester.

Justus is known to have written to the British and Irish Christians, asking them to conform to the ways of the Church of Rome. An extract from one of his letters is included in Bede. He does not come across as terribly tactful and the letter was largely ignored. The most notable event of his brief archiepiscopate was the evangelization of Northumbria. Paulinus was consecrated by Justus to be the first Archbishop of York and, within two years, King Edwin of Northumbria was baptised, with many of his people, in a little church which he had built at York, near where now York Minster, stands. The good news was conveyed to Justus not long before his death, which is believed to have taken place in 627.