Two suspects forced entry into the main building in the early hours of Monday morning, with staff on duty overnight uninjured.
Objects including Bronze Age gold jewelry have reportedly been stolen from St Fagans National Museum of History.
Police say they received a report on 12.30am on Monday, October 6, and believe two suspects forced entry into the main building.
Detective Inspector Bob Chambers, from South Wales Police, said: “At this time, we believe that two suspects forced entry to the main building, where several items, including Bronze Age gold jewellery, have been stolen from a display case.
“An investigation is on-going, and we urge anyone with information to contact us as soon as possible.
The museum is operated by Amgueddfa Cymru. A spokesperson said the museum was “saddened by the events in what was a targeted attack on St Fagans National Museum of History in Cardiff.
“The objects presumed stolen are examples of Bronze Age gold jewellery and were on display in the Wales gallery.”
“We would like to thank South Wales Police for their cooperation in this investigation and their prompt response when they were alerted in the early hours.
“We are also very grateful to the staff members who were on duty overnight, followed protocol and who were thankfully uninjured during the incident.”
Anyone with information is asked to contact South Wales Police quoting 2500319252.
