Isigonus of Nicaea was a paradoxographer from the city of Nicaea in Bithynia. He is dated to the 1st–2nd century CE based on citations by later authors like Phlegon of Tralles. No details of his personal life or education survive.

His only known work is the lost Apista, or "Unbelievable Things." This collection of marvels belonged to the paradoxographical tradition, cataloging strange phenomena and monstrous creatures. Phlegon of Tralles cites it as a source for tales of hermaphrodites and ghostly apparitions.

Isigonus’s significance lies in his contribution to paradoxography. His Apista served as a source for later compilers, reflecting the period's intellectual curiosity about the boundaries of the natural world. The work represents a blend of geography, ethnography, and folklore distinct from mainstream historical writing.