The Segontiaci tribe were not mentioned by the second century geographer Ptolemy, but were thought to be a sub-sect of the Ordovices.
There may have been a minor tribe named the Segonti, Segontii or even Segontiaci in the area of modern Caernarfon, and have lent their name to the nearby Roman fort and minor settlement of Segontivm. There is mention of a Segontiaci tribe in Southern England in the memoirs of Julius Caesar, in the mid first century BC; but the relationship - if any existed - between the two tribes is unknown.
Concerning the ancestry of the Segontii, we are given doubtful information by Julius Caesar:
"When the Trinobantes¹ had been placed under protection and secured from all outrage at the hands of the troops, the Cenimagni, the Segontiaci, the Ancalites, the Bibroci and the Cassi² sent deputations and surrendered to Caesar. ..." [54BC] (Caesar De Bello Gallico v.21)
The Segontiaci of Caesar may possibly be identified with the Segontium region of North Wales, though less likely, they may have been Celtiberian settlers originally from the Caesaraugustanus district of Hispania Tarraconensis (Northern Spain), where there are important towns named Segontia (Sigüenza), Segovia (near Madrid) and Segobriga (near Tarancón), also nearby Saguntum, the modern Sagunto, a Mediterranean coastal town on the Golfo de Valencia.