It was photographed by a council worker.
A European Swallowtail butterfly was recently spotted and photographed in Rougemont Gardens.
Andrew Thomas spotted the butterfly on a Mulberry tree.
He said: “It had attracted a few admirers, which was what initially prompted us to investigate what was the subject of their interest.
“I wasn’t expecting that beauty, they really are a large butterfly. I saw them when I went to Corfu some years ago.
“I reported it to the Devon branch of the Butterfly Conservation Group. Apparently it’s their second sighting this year, the other was found in Barnstaple.”
Steve Hussey, of the Devon Wildlife Trust, said: “This butterfly is usually only found in the UK in Norfolk, so to see it in Devon is a real rarity.
“It is possible that this butterfly and the other recent Devon sighting at Barnstaple were either escapees from butterfly collections or else a deliberate release.
“However, this is a large and strong flying butterfly, so it is also possible that one or both of these sightings were naturally occurring vagrants from continental Europe.”