Longinus (Greek: Λογγίνος) is the name of the Roman soldier who pierced the side of Jesus with a lance, who in apostolic and some modern Christian traditions is described as a convert to Christianity. His name first appeared in the apocryphal Gospel of Nicodemus. The lance is called in Catholic Christianity the "Holy Lance" (lancea) and the story is related in the Gospel of John during the Crucifixion. This act is said to have created the last of the Five Holy Wounds of Christ.

This person, unnamed in the Gospels, is further identified in some versions of the story as the centurion present at the Crucifixion, who said that Jesus was the son of God, so he is considered as one of the first Christians and Roman converts. Longinus's legend grew over the years to the point that he was said to have converted to Christianity after the Crucifixion, and he is traditionally venerated as a saint in the Roman Catholic Church, Eastern Orthodox Church, and several other Christian communions.

No name for this soldier is given in the canonical Gospels; the name Longinus is instead found in the apocryphal Gospel of Nicodemus. Longinus was not originally a saint in Christian tradition. An early tradition, found in a sixth- or seventh-century pseudepigraphal "Letter of Herod to Pilate", claims that Longinus suffered for having pierced Jesus, and that he was condemned to a cave where every night a lion came and mauled him until dawn, after which his body healed back to normal, in a pattern that would repeat until the end of time. Later traditions turned him into a Christian convert, but as Sabine Baring-Gould observed: "The name of Longinus was not known to the Greeks previous to the patriarch Germanus, in 715. It was introduced among the Westerns from the Apocryphal Gospel of Nicodemus. There is no reliable authority for the Acts and martyrdom of this saint."

The name is probably Latinized into a common cognomen of the Cassia gens, from the Greek lónchē (λόγχη), the word used for the spear mentioned in John 19:34. It first appears lettered on an illumination of the Crucifixion beside the figure of the soldier holding a spear, written, perhaps contemporaneously, in horizontal Greek letters, LOGINOS (ΛΟΓΙΝΟϹ), in the Syriac gospel manuscript illuminated by a certain Rabulas in the year 586, in the Laurentian Library, Florence. The spear used is known as the Holy Lance, and more recently, especially in occult circles, as the "Spear of Destiny", which was revered at Jerusalem by the sixth century, although neither the centurion nor the name "Longinus" were invoked in any surviving report. As the "Lance of Longinus", the spear figures in the legends of the Holy Grail.^{[citation needed]}

Blindness or other eye problems are not mentioned until after the tenth century. Petrus Comestor was one of the first to add an eyesight problem to the legend and his text can be translated as "blind", "dim-sighted" or "weak-sighted". The Golden Legend says that he saw celestial signs before conversion and that his eye problems might have been caused by illness or age. The touch of Jesus's blood cures his eye problem:

Christian legend has it that Longinus was a blind Roman centurion who thrust the spear into Christ's side at the crucifixion. Some of Jesus's blood fell upon his eyes and he was healed. Upon this miracle Longinus believed in Jesus.

The body of Longinus is said to have been lost twice, but discovered at Mantua, together with the Holy Sponge stained with Christ's blood, wherewith it was told—extending Longinus's role—that Longinus had assisted in cleansing Christ's body when it was taken down from the cross. The relic enjoyed a revived cult in the late 13th century under the patronage of the Bonacolsi.

The relics are said to have been divided and then distributed to Prague (St. Peter and Paul Basilica, Vyšehrad) and elsewhere. Greek sources assert that he suffered martyrdom in Cappadocia. The Russian Orthodox Cathedral of St. John the Baptist, Washington, DC, purports to have a holy relic, a fragment of bone, of Saint Longinus.