Damnonia may refer to:

• Damnonia, land of the Damnonii, a Celtic tribe of Roman Britain in what today is southern Scotland
• Damnonia, a name of the Kingdom of Strathclyde, the early mediaeval kingdom that subsumed the Damnonii
• Damnonia, an alternative spelling of Dumnonia, the early mediaeval kingdom (named for the Celtic tribe of Roman Britain, the Dumnonii), in what today is Devon and Cornwall.

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