Thieves have targeted the former coastguard building in Mumbles and stripped it of lead.
The building on Mumbles Head was repeatedly visited last week by people who took away large amounts of lead from the property.
The headquarters closed in 2015 but the Maritime and Coastguard Agency said the facility is still used to assist in the coastal operations work of HM Coastguard.
It added the MCA’s estates team is aware of the damage to the building.
A coastguard source said it was believed the theft had taken place over three consecutive nights.
A spokesman for South Wales Police said: “Police in Swansea are investigating an incident of vandalism upon the HM Coastguard building in Mumbles.
"Sometime between Tuesday, November 23, and the following day, damaged was caused to the exterior of the building.
"Anyone with information is asked to contact South Wales Police on 101 quoting 2100412455.”
The closure of the coastguard was announced in 2011, prompting a 110,000 calling for it to be saved, but it closed in 2015.
Two years ago the MCA suggested alternative uses for the building, and asked Swansea Council to to consider an 'exploratory' request, with options including a hotel, housing, or offices.
The station at Tutt Head on Bracelet Bay, lies within the Gower Area of Outstanding Natural Beauty.
Get Swansea stories straight to your inbox with our newsletter.
