The Pagan Society is offering a reward of £1,000 for the capture of the person who daubed yellow paint all over the historic Rollright Stones near Chipping Norton.
The attack happened last week when every stone in the circle was vandalised.
The Rollright Stones, near Chipping Norton, are the third most important stone circle in the country after Stonehenge and Avebury, both in Wiltshire.
The circle, which comprises about 70 stones, is known as the King's Men, while another five nearby are known as the Whispering Knights. On the other side of the road is the King Stone. They are believed to be around 3,500 years old.
English Heritage sent an expert who has dealt with similar damage at Avebury to examine the stones and is deciding how to remove the paint.
Curator Don Prout said: "This was a methodical and systematic attack. There is paint each side of every stone.
"The paint is now being analysed. The Pagan Society has offered the money for the arrest and conviction of whoever did it."
Anyone with information should call Thames Valley Police on 08458 505505.
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