Codex Alexandrinus is one of the four Great uncial codices and among the earliest and most complete manuscripts of the Bible. It is the oldest manuscript to use larger letters to indicate new sections, and it was the first manuscript of great importance and antiquity to be extensively used by textual critics.

It is a palimpsest where the original 5th-century biblical text was washed off and overwritten in the 12th century with treatises by Ephrem the Syrian. It is one of the four great uncials, and its underlying text was famously deciphered by Constantin von Tischendorf in the 1840s.