The Gewisse Latin: Gewissæ) were a political group^[a] or band of Anglo-Saxons. They originally settled in the upper Thames river valley area. The Gewisse were the most powerful group in what would become the kingdom of Wessex.

The name 'Gewisse' comes from an Old English word for "reliable" or "sure",^[1] In the genealogy of King Alfred of Wessex, is a supposed ancestor of Cerdic named Gewis.^[2] (also spelled Gwis).^[3] The true origin of the name cannot be determined accurately.^[4] the name was old in Bede's time. Bede stated the original name for the West Saxons was the Gewisse.^[5] The change in name from the Gewisse to the West Saxons seems to have taken place in the latter part of the 7th century.^[1] The Gewisse, like the 'Cantwara' (Kent-men) and the 'Meonwara' of Hampshire were political units, not tribes.^[6]

In 577 the Anglo-Saxon Chronicle claims the kings of the Gewisse captured Cirencester, Bath and Gloucester.^[7] The date is questionable but the Gewisse did hold those territories.^[7] The Gewisse found themselves limited in their ability to expand into new territories by the growing power of Mercia. The Anglo-Saxon Chronicle for 628 says that Cyngils and Cwichelm his son 'fought with Penda at Cirencester and came to an agreement with him there'.^[8] The 'agreement' meant giving Cirencester to Penda.^[9] It was a permanent loss of Cirencester.^[10] By the second half of the 7th century Gewisse was expanding towards the Southwest and Devon.^[11] At about the same time they lost territory in the Thames valley to Mercia. The change in name to Wessex may have reflected more a change in their territory.^[11]

Bishop Birinus began converting the Gewisse to Christianity in 634, In 635 he baptized their king Cynegils who established Birinus at Dorchester.^[10] But the Dorchester bishopric was never considered West Saxon by Bede.^[11] He considered it to be Gewisse. In 660 Cenwalh created a new diocese at Winchester and made Wine the first bishop. Bede referred to the Winchester diocese as West Saxon.^[11] At the time he was writing in the early 8th century, Bede treated the two names as interchangeable.^[12] But when writing of the reign of Cynegils he referred to them as "anciently known as the Gewissae."^[13]