Gaius (/ˈɡaɪ.əs/ or /ˈɡeɪ.əs/) is a Latin praenomen, or personal name, which was one of the most common names throughout Roman history. The feminine form is Gaia. The praenomen was used by both patrician and plebeian families, and gave rise to the patronymic gens Gavia. The name was regularly abbreviated C., based on the original spelling of Caius, which dates from the period before the letters "C" and "G" were differentiated.

Throughout Roman history, Gaius was generally the second-most common praenomen, following only Lucius. Although many prominent families did not use it at all, it was so widely distributed amongst all social classes that "Gaius" became a generic name for any man, and "Gaia" for any woman. A familiar Roman wedding ceremony included the words, spoken by the bride, ubi tu Gaius, ego Gaia ("as you are Gaius, I am Gaia"), to which the bridegroom replied, ubi tu Gaia, ego Gaius. The name survived the collapse of the Western Empire in the 5th century, and continued into modern times.