Guildayichthys is an extinct genus of marine fish that belonged to the order Guildayichthyiformes that lived in Montana during the Mississippian. It is a monospecific genus with the only species in this genus being Guildayichthys carnegiei. It has a highly compressed, discoidal body. It also has short teeth.

It was named after John Guilday who was a curator at the Fossil Mammals of Carnegie Museum in Pittsburgh, Pennsylvania. It was named after Guilday for his "unique appreciation of the beauty of life".