Regillum or Inregillum was a town in ancient Sabinum, north of Rome, known chiefly as the original home of Appius Claudius Sabinus Regillensis.
In 504 BC, as the majority of the Sabines seemed ready to vote for war, Clausus and his retainers migrated to Rome, where they were warmly received.
His followers, numbering some five hundred men capable of bearing arms, were granted land north of the Anio, where they later formed the basis of the tribus Claudia.
The Battle of Lake Regillus was one of the most important events in the early Republic, as a Roman army under the command of the dictator Aulus Postumius Albus defeated an alliance of Latin towns, led by Octavius Mamilius, the dictator of Tusculum, which aimed to restore Lucius Tarquinius Priscus, the seventh and last King of Rome, to the throne.
Postumius obtained the surname Regillensis as a result of his victory; Claudius presumably received it as a native of the town, although it is possible that he also participated in the battle.