The owl was found on marshes at the former Bettisfield colliery in Bagillt.
RSPCA inspector Jenny Anderton said the bird had no major injuries but was "extremely thin and just needs time to fully recuperate".
The bird is now a rarely seen guest at the Stapeley Grange Wildlife Centre - where just five of the owls have been treated in the last decade.
A "rare" short-eared owl found emaciated in Flintshire is now recuperating well, the RSPCA have said.