Since he refused to sacrifice to the Roman gods, he was found guilty of sacrilegium and sentenced to death by drowning.

According to a local tradition, he was thrown from a small boat into the Gulf of Trieste, off of the present-day promontory of Sant'Andrea.

Sebastian gathered his fellow believers and they went searching for the body, which they found on what is today Riva Grumula.

In late antique times the area near Piazza Hortis in Trieste was a cemeterial one and there is a good possibility that the former basilica of the Holy Martyrs at the corner of Via Ciamician and Via Duca d'Aosta was built on Justus' tomb.

In the Middle Ages, the body of Justus was translated to a chapel adjacent to the church of Mary Mother of God (the present-day Duomo), attested since the 6th century.