The Roman statesman and philosopher Anicius Manlius Seuerinus Boethius (c. 480–c. 525) translated and wrote commentaries on most of Aristotle's logical works. In addition, he wrote several treatises on logic, including two monographs on the categorical syllogism, which are commonly known as De syllogismo categorico and Introductio ad syllogismos categoricos. The present study is the first critical edition of the former. De syllogismo categorico divides into two books of which the first is an account of the categorical proposition and the second deals with the categorical syllogism. The ultimate sources are Aristotle's Peri hermeneias and Analytica priora, but certain dispositional, terminological, and doctrinological features show that the text is heavily influenced by the tradition of the Greek commentators. From the rediscovery of Boethius' logical writings in the 10th century until the mid-12th century, Aristotle's doctrine of the categorical syllogism was known chiefly through Boethius’ De syllogismo categorico. The influence by as well as on the work is discussed in the introduction to the present study. The reconstitution of the text rests on collation of 44 manuscripts dating from the 10th to the 15th century. An analysis of the interrelation of the manuscripts leads to the conclusion that all extant manuscripts descend from a common archetype but that the tradition is severely contaminated and cannot be described by means of a stemma codicum. The text is primarily based on the fifteen earliest text witnesses, among which a formal hierarchy is established. The Latin text is presented with a critical apparatus, an apparatus fontium, a translation, and notes. Variant readings in the later manuscripts are reported in an appendix.