The odd image was first spotted on 20 animals grazing in a field in Melton Mowbray, Leicestershire.
There have since been sightings in a further five fields, from Perthshire in Scotland to Exeter, 500 miles away in Devon
Photos of the sheep have gone viral on Twitter, with users sharing sightings and speculating about the source of the smiley face – using the hashtag #creepysheep.
Pranksters are being blamed for the bizarre branding – but some internet users are convinced aliens are responsible.
Farmers, meanwhile, are scratching their heads – wondering why their flocks have been targeted.