Codex Usserianus Primus (Dublin, Trinity College Library, 55) is an early 7th Century Old Latin Gospel Book. The manuscript is damaged, with the leaves being fragmentary and discolored. The remains of the approximately 180 vellum folios have been remounted on paper.

The manuscript has a single remaining decoration, a cross outlined in black dots at the end of the Luke (fol. 149v). The cross is between the Greek letters alpha and omega. It is also flanked by the explicit for Luke and the incipit for Mark. The entire assemblage is contained within a triple square frame of dots and small "s" marks with crescent shaped corner motifs. The cross has been compared to similar crosses found in the Bologna Lactantius, the Paris St. John, and the Valerianus Gospels. Initials on folios 94, 101 and 107 have been set off by small red dots. This represents the first appearance of decoration by "dotting" around text, a motif which would be important in later Insular manuscripts.

There are some paleographic similarities with early manuscripts produced at the monastery at Bobbio, such as the Bobbio Jerome and the Bobbio Orosius. However, it is now thought to have been produced in Ireland. It may therefore be the earliest surviving Irish codex.

The manuscript's traditional name can be translated as "the First Book if Uss(h)er" and refers to James Ussher, Archbishop of Armagh. Despite the name, it is doubtful that Ussher ever actually owned the manuscript.