Firmicus may refer to:
- The lunar crater Firmicus. This is a list of craters on the Moon. The large majority of these features are Impact craters The crater nomenclature is governed by the International Firmicus is a lunar crater that lies in the eastern part of the Moon 's near side so that from Earth it appears oval in shape due to foreshortening
- The Christian astrologer Julius Firmicus Maternus (fourth century), after whom the crater is named. Christianity ( Greek Χριστιανισμός from the word Xριστός ( Christ)is a monotheistic Religion centered on the life and teachings Julius Firmicus Maternus was a Christian Latin writer and notable Astrologer, who lived in the reign of Constantine I and his successors
The spider genus Firmicus (Thomisidae). Crab spiders make up the Thomisidae family of the Araneae order