The Battle of Peonnum was fought approximately AD 660 between the West Saxons under Cenwalh and the Britons of what is now Somerset. It was a decisive victory for the Saxons, who gained control of Somerset as far west as the River Parrett. The location of the battle is uncertain.
The border between the West Saxons and the Britons of Somerset had been established at the Wansdyke along the ridge of the Mendip Hills following the Battle of Deorham and the occupation of Bath in 577. Then in 652, Cenwalh broke through at the Battle of Bradford on Avon.
Relief for the Britons came when Cenwalh was exiled to East Anglia after a squabble with Penda of Mercia. Some time after his return he renewed the attack on the British tribes and in 658 his army met the Britons for a climatic battle at Peonnum. The Saxons were victorious, and Cenwalh advanced west through the Polden Hills to the River Parrett, annexing eastern and central Somerset. The territory gained was modest in size; Geoffrey Ashe suggests that Cenwalh's ultimate goal may have been gaining control over the valuable Glastonbury Abbey located within it.
The border stabilised at the Parrett until 681-685, when Centwine of Wessex defeated King Cadwaladr of Gwynedd and his local allies, allowing them to occupy the rest of Somerset west and north to the Bristol Channel. West Saxon rule was consolidated and further extended into Devon by King Ina.
The battle is stated to have happened æt peonnum, which means "at the penns". Penn is the Celtic word for "head" or "top", which here is likely used for "hill" or "peak". Suggested locations include Penselwood (Pen Selwood), near Wincanton which is called Penna in the Domesday Book, Pinhoe or Pen Beacon in Devon, and Penn (near Yeovil).