The Onomasticon (Ancient Greek: Ὀνομαστικόν, Onomastikón), more fully On the Place Names in the Holy Scripture (Περὶ τῶν Τοπικῶν Ὀνομάτων τῶν ἐν τῇ Θείᾳ Γραφῇ, Peri tōn Topikōn Onomatōn tōn en tē Theia Graphē), is a gazetteer of historical and then-current place names in Palestine and Transjordan compiled by Eusebius (c. AD 260/265–339), bishop of Caesarea, and traditionally dated to sometime before 324.

The Onomasticon sits uneasily between the ancient genres of geography and lexicography, taking elements from both but serving as a member of neither. It is widely considered the most important book for the study of Palestine in the Roman period. Its influence can be detected both in the Madaba map and the accounts of early Christian pilgrims, and it most probably contributed to the Christian pilgrimage of the 4th century, constructing "The Holy Land" as a unifying idea for Christians. Even so, it appears that the Onomasticon was not meant to be a guide for pilgrims, as it did not mention places to be venerated, rather, its target audience was biblical scholars and the composition was meant as an exegetical tool for understanding scripture.

Eusebius's description of his own method, who wrote: "I shall collect the entries from the whole of the divinely inspired Scriptures, and I shall set them out grouped by their initial letters so that one may easily perceive what lies scattered throughout the text," implies that he had no similar type of book to work from; his work being entirely original, based only on the text of the Bible. Some have stated that, based on the precise distances and directions he gave, Eusebius himself visited the locations he wrote about in many cases, though he explicitly relied on other testimonies at times, while sites with comparatively more complete descriptions may have been seen by Eusebius himself or a loyal informant, and the little information on concise entries may have been gleaned from scripture alone. Others have suggested that Eusebius had at his disposal early Roman maps of the Roman Empire with which to work, and which allowed him to record the precise distances between locations in Roman miles. In almost all of the entries in his geographical opus, Eusebius brings down the respective distances in Roman "milestones" (semeia) from major points of reference, such as from Jerusalem, Beit Gubrin (Eleutheropolis), Hebron, Ptolemais, Caesarea, etc. However, in the Onomasticon distances between each "milestone" were usually 1,600 meters–1,700 meters, while the standard Roman mile was 1,475 meters, and since most villages in the Onomasticon are far removed from Roman-built roads, scholars have concluded that Eusebius did not glean the geographical information from maps based on a milestone survey, but rather collected the information from some other source. Needless to say, this innovation has been very useful to modern research. Of the approximate 980 Biblical and New Testament names of places contained in those works, Eusebius identifies some 340 with locations known in his own day and age.

From the preface to the work it is apparent that Eusebius received guidance from Paulinus on the ways it may be improved and made worthy of publishing. Paulinus may have suggested that Eusebius add references to the New Testament, but if so Eusebius was not interested in completing the job, as only a few Gospel sites were mentioned in the work and Jesus is entirely absent from the entry on Bethlehem. A number of scholars have offered explanations for the rarity of references to the New Testament in the Onomasticon. Melamed suggested that Eusebius focused mainly on the sites mentioned in Hebrew scripture because he was using a Jewish written source. However, Taylor noted that early Christian exegetes as well as later pilgrims were wholly more focused on the Old Testament, thus sites such as Golgotha and Akeldama were added in the final stage of composition, possibly per suggestion of Paulinus and reflected in the end of the preface where Eusebius wrote that he would collect names "from the whole of divinely inspired scripture" (2:17-18) an assertion he would not substantiate.

The original scope of the work included three sections:

The latter two sections were lost, but the fact that they concern matters from the Old Testament confirms that the surviving section was originally intended to encompass the place names from the Old Testament alone, with the New Testament as an afterthought.

In various entries, Eusebius compares different variants of Greek scriptural text and it is assumed that he used Origen's Hexapla as his master text. Of the six versions compared in the Hexapla, Eusebius used Origen's version of the Septuagint, which appeared in column 5 of the text, as the standard, and the variants from the other columns he referred to by using Greek abbreviations for each.

The place names in the Onomasticon are arranged alphabetically, according to the books in the Septuagint. It appears that the text was meant to be used as a reference, while studying a particular biblical book and upon encountering a place name beginning with a certain letter, the reader was expected to find the section with the letter, find the book they were studying and then locate the place-name within the work. The list begins with place names found in the Pentateuch, excluding Leviticus. It then proceeds to the books of Joshua, Judges, Samuel, Kings, 1 Chronicles, Isaiah, Hosea, Zechariah, Micah, Ezekiel, Amos, Jeremiah, Job, and 1 Maccabees (collectively referred to "Kingdoms"). The final entries are places mentioned in the Gospels, though some of these were appended to the listings under Kingdoms. The place names mentioned in Psalms, the Song of Songs, Obadiah, Joel, Jonah, Nahum, Zephaniah, Esther, Daniel, Ezra, Nehemiah, Habakkuk, Haggai, Malachi, Ruth, Proverbs, Ecclesiastes and Lamentations are absent, possibly because Eusebius did not have sufficient resources to supply more information about them. A minority of the entries are based on Eusebius's own observations, while the majority rely solely on textual sources and are notably concise.