Police are appealing for information after the Roman amphitheatre in Caerleon was vandalised over the bank holiday weekend.
Graffiti has been scrawled on the remains of the Roman shrine, near Newport, in the latest in a number of incidents of vandalism at the monument in recent months.
Earlier this year Gwent Police investigated several incidents where stones were removed from the walls of the 2,000-year-old ancient monument and "thrown around", causing damage to the structure and its archaeology.
At the time, police warned that continued vandalism of the ruins would be "catastrophic" and said the damage could be "unmeasurable" due to its rich history.
Police are now seeking information on who may have carried out the vandalism.
A statement from Gwent Police said: "Heritage crime reported at Caerleon Amphitheatre where graffiti has been scrawled on the remains of a Roman shrine, log number 209 31/08/20.
"Any information on those responsible or concern when people are on site outside of opening hours, please call 101 to report."
Cadw has been contacted for comment.