Stone has been removed from the site of Dun Torcuill - a broch on an uninhabited island on a loch on North Uist.
The dun, a scheduled monument, is considered to be the finest and best-preserved example of its type across both Uists and Benbecula.
Sergeant Gavin McDevitt, from Lochmaddy police station, said: "Damaging or removing any part of a scheduled monument is a criminal offence. In this instance, there is the potential for destabilisation of the monument, and/or the loss of archaeological deposits previously protected by the tumbled stone.
"It is an offence under the Ancient Monuments and Archaeological Areas Act 1979 to undertake works without scheduled monument consent.
"The penalties for offences under the act are considerably higher than those for damage to property or vandalism offences, with fines ranging up to £50,000 or prison sentences of up to two years.
"If anyone has any information that could help, please call 101."