The Cornovii were part of the Dumnonii, a Celtic tribe who inhabited the South West peninsula of Great Britain, during the Iron Age, Roman and post-Roman periods. (4th century BC-343 AD).  The Cornovii are assumed to have lived at the western end of the peninsula, in the area now known as Cornwall, and the name is the ultimate source of the name of that present-day county.
The existence of this sub-tribe, clan, or sept is not mentioned in Ptolemy's 2nd century Geography, as are many of the other Iron Age tribes in Britain, but it has been inferred from a place-name listed in the Ravenna Cosmography of c.700 AD as purocoronavis, which is considered to be a scribal error for durocornavis (or durocornovium), meaning "the fortress of the Cornovii". The location of Durocornavis has not been identified, but it may possibly have been at Tintagel or Carn Brea.
According to Ptolemy, there were two other tribes known as the Cornovii, one in the Midlands and one in Northern Scotland. The etymology of the tribal name is uncertain: although it is accepted that *corn literally means "horn", there is disagreement over whether or not this refers to the shape of the land. Considering that Cornwall is at the end of a long tapering peninsula, many scholars have adopted this derivation for the Cornish Cornovii. For instance, Graham Webster in The Cornovii (1991), which is primarily about the Midlands tribe, states that this could apply as long as the geography was apparent, as it might have been to Roman surveyors in the first century, and Victor Watts in the Cambridge Dictionary of English Place-names (2010) derives the name from a postulated original tribal name *Cornowii, "the people of the horn".
Malcolm Todd, in The South West to AD 1000 (1987), discusses other etymologies that have been put forward, such as the name being a reference to dwellers in promontory forts, and an explanation hypothesised by Ann Ross in 1967 that the tribal names may be totemic cult-names referring to a "horned god" cult followed by the tribes, which Todd says may be cognate with the Gaulish Cernunnos or the unnamed horned god of the Brigantes.
In an attempt to explain the same tribal name being used in the Midlands and Cornwall, the historian John Morris put forward a theory in his work The Age of Arthur (1973) that the Cornovii from the West Midlands migrated to Cornwall in 460 AD. Morris suggested that a contingent was sent to Dumnonia to rule the land there and keep out the invading Irish seeing that a similar situation had occurred in North Wales. Morris's theory is not generally accepted by modern scholarship: Philip Payton, in his book Cornwall: a history (2004), states "...the Morris thesis is not widely accepted by archaeologists and early historians, and we may safely conclude that the Cornovii located west of the Tamar were an indigenous people quite separate from their namesakes in the Midlands and Caithness."
The extreme western peninsula of Dumnonia came to be known as "Cerniw" in Welsh, "Kernow" in Cornish and "Kernev (Veur)" in Breton.[when?] The Celtic root word corn- (horn) and the suffix wealas being the Anglo-Saxon word, meaning "foreigner", (which was also applied to the Welsh), hence the Anglo-Saxon name of Corn-wealas, that may mean the "welsh/foreigners of the horn". Although this theory is widely acknowledged it is also problematic.
Since the Cornovii are only known from one mention in antiquity, nothing is known for certain of their history. They were part of the Dumnonii, the tribe whose lands, known as Dumnonia, extended from Cornwall through Devon and included parts of Somerset and Dorset. For details of the people who lived in the area after the withdrawal of the Romans, see History of Cornwall. After the passing of the Roman period they re-appeared in 430 AD as a sub-Dukedom of Dumnonia until 875 AD. The Dukedom became independent outright until 1066 AD.
The Dumnonii had no known tribal centre, and although Ptolemy's Geography lists four places as Dumnonian poleis: Voliba, Tamara, Uxella and Isca Dumnoniorum (present day Exeter), it is likely that he only listed Roman places, not purely native sites. Of these, there has been speculation that Voliba might be a place in Cornwall and Tamara is assumed to be on the River Tamar, the border between Cornwall and Devon.
Ptolemy's list is supplemented by the problematical Ravenna Cosmography, which lists sixteen names before Isca Dumnoniorum (which is listed as scadum namorum, showing some of the problems of this source) and which have therefore been assumed to be west of Exeter. Several have been identified as most likely in Devon, leaving purocoronavis (the source of the name Cornovii, as discussed above), and a few others that are so corrupt as to defy identification, such as Giano, Pilais, and Vernalis, which may possibly refer to places within the lands occupied by the Cornovii.
- Payton, Philip (2004). Cornwall: a history (revised ed.). Fowey: Cornwall Editions Ltd. ISBN 1-904880-00-2.
- "Celtic Kingdoms of the British Isles Celts of Britain". 2014. The History Files. Retrieved 13 June 2014.
- Charles Thomas. (1986). Celtic Britain. Ancient Peoples & Places Series. London: Thames & Hudson
- Morris, John (1973) The Age of Arthur
- Todd (1987), p. 203.
- Webster, Graham (1991). The Cornovii. Peoples of Roman Britain (revised ed.). Alan Sutton. pp. 19, 21. ISBN 0-86299-877-8.
- Rivet & Smith (1979). pp. 324–5
- Watts, Victor (2010). The Cambridge Dictionary of English Place-names (1st paperback ed.). Cambridge University Press. p. 158. ISBN 978-0-521-16855-7.
- Todd (1987). p. 217.
- Todd (1987). p. 202.
- Rivet & Smith. (1979) pp. 367–8. (see "Glanum")
- Rivet & Smith. (1979) p. 440.
- Rivet & Smith. (1979) pp. 494–5.
- Rivet, A. L. F.; Smith, Colin (1979). The Place-names of Roman Britain. London: Batsford Ltd. pp. 324–5. ISBN 0-7134-2077-4.
- Todd, Malcolm (1987). The South West to AD 1000. A Regional History of England. Longman. ISBN 0-582-49274-2.