The Battle of the Boarn ( ) was an eighth century battle between the Franks and the Frisians near the mouth of the river Boarn in what is now the Dutch province of Friesland. In 734 a Frankish army commanded by Majordomo Charles Martel invaded Friesland in a campaign that was part of a series of ongoing wars and skirmishes between the Franks and the Frisians. Marching along the river Boarn the Frankish army reached the mouth of the river where it used to flow into the Bordine estuary or Middelsee. This estuary has since silted up and been claimed for agriculture during the 10th to 14th century.