The Battle of Maserfield (or Maserfeld, "marsh (border) field"; Welsh: Maes Cogwy), is fought on 5 August 641 or 642, between the Anglo-Saxon kings Oswald of Northumbria and Penda of Mercia, ending in Oswald's defeat, death, and dismemberment.

The battle is also known as Cogwy to the Welsh, with their countrymen from Pengwern participating in the battle (according to the Canu Heledd), probably as allies of the Mercians.

Bede reports the commonly accepted date given above; the Welsh Annales Cambriae is generally considered incorrect in giving the year of the battle as 644.

The site of the battle is traditionally identified with Oswestry; arguments have been made for and against the accuracy of this identification.