Codex Koridethi, also named Codex Coridethianus, is a Greek uncial manuscript of the New Testament Gospels, written on parchment. It is designated by the siglum Θ or 038 in the Gregory-Aland numbering of New Testament manuscripts, and as ε050 in the Soden numbering of New Testament manuscripts. Using the study of comparative writing styles (palaeography), it has been assigned to the 9th century CE. The manuscript has several gaps. It has been considered by scholars as a leading member of the Caesarean text-type in the Gospel of Mark.

The manuscript is a codex (precursor to the modern book format), containing an almost complete text of the four Gospels written on 249 parchment leaves (size 29 cm by 24 cm), with the following gaps: Matthew 1:1–9, 1:21–4:4, and 4:17–5:4. The text is written in two columns per page, with 19-32 lines per column. The letters are written in a rough, inelegant hand in blackish-brown ink. Greek accents (used to indicate voiced pitch changes) are written, but breathing marks (utilised to designate vowel emphasis) are rarely included.

The scribe who wrote the text is believed to have been unfamiliar with Greek. The manuscript includes the Ammonian sections, but not always the Eusebian Canons, both early systems of dividing the four Gospels into different sections. Lectionary (weekly church reading portions) beginning (αρχη / arche) and ending (τελος / telos) marks are also written.

Quotations from the Old Testament are marked. The tables of contents (known as κεφαλαια / kephalaia) are included before the gospels of Mark, Luke, and John (it is unable to confirm whether there was a contents-table before Matthew, as the manuscript is missing up to Matt 1:9). A brief subscription is written after the Gospel of John ends.

The Greek text of the Gospel of Matthew chapters 1-14, and the whole of the Gospel of Luke and Gospel of John is considered to be more or less a representative of the Byzantine text-type, while the text of the Gospel of Mark has been considered to be a representative of the Caesarean text-type. The text-types are groups of different New Testament manuscripts which share specific or generally related readings, which then differ from each other group, and thus the conflicting readings can separate out the groups. These are then used to determine the original text as published; there are three main groups with names: Alexandrian, Western, and Byzantine. 

The Caesarean text-type, initially identified by biblical scholar Burnett Hillman Streeter, has been contested by several text-critics, such as Kurt and Barbara Aland. The text of Matthew chapters 14-28 has been considered to be a representative of the Alexandrian text-type. Aland placed it in Category II of his New Testament manuscript classification system. Category II manuscripts are described as being manuscripts "of a special quality, i.e., manuscripts with a considerable proportion of the early text, but which are marked by alien influences. These influences are usually of smoother, improved readings, and in later periods by infiltration by the Byzantine text." It lacks the text of the Pericope Adulterae (John 7:53-8:11).

(See main article the Caesarean text-type)

Streeter based his identification of a new text-type primarily on the readings found on this codex in the Gospel of Mark, and their corresponding appearances in the biblical citations in the writings of the early church father, Origen. He also grouped the manuscripts of ƒ¹, ƒ¹³, and the minuscules 28, 565 and 700 along with Codex Koridethi, initially designating them as fam. Θ.