Ferrex was a legendary king of the Britons as accounted by Geoffrey of Monmouth. He was the son of Gorboduc and Judon.

When his father had become old, he waged war on his brother, Porrex, for who would succeed to the kingship. He fled to Gaul and enlisted the help of Suhard, the king of the Franks. Upon his return, he fought a battle with his brother and died there. The Gaulish army was also destroyed. His mother, Judon, avenged his death by killing Porrex in his sleep some time later. This led to a civil war for some time in Britain.