Theodosius of Alexandria was a Greek grammarian active in the late fourth and early fifth centuries CE. He worked within the scholarly tradition of Alexandria, though no specific biographical details about his life or education survive.

His principal work is the Canones peri kliseōs onomatōn kai rhēmatōn, a systematic treatise on the inflection of Greek nouns and verbs. This grammatical handbook was preserved and used extensively in later Byzantine education.

Theodosius's Canones is a key work of Late Antique Greek grammatical scholarship. It provided a concise set of morphological rules that served as a standard textbook for centuries. Its influence endured through the medieval period, with later grammarians such as Maximus Planudes compiling and commenting on its precepts.