Isaeus (fl. early 4th century BC) was a Greek orator. Traditionally, he was born at Chalcis in Euboea around 420 BC. He came to Athens as a student of Isocrates and later taught Demosthenes whilst working as a metic speechwriter for others. He was one of the ten great Attic orators although only eleven of his speeches survive, with fragments of a twelfth. They are mostly concerned with inheritance, with one on civil rights.

Dionysius of Halicarnassus compared his style to Lysias although Isaeus was more given to employing sophistry.