Anti-English graffiti was daubed on an outbuilding at a farm on the Narberth Bridge to Molleston road overnight on Friday.
Red and cream paint was used in the incident which saw the words 'Troi Nol Saes', which translates as 'turn back English', daubed on the building.
Also painted were the initials FWA, which stands for Free Wales Army, MG, representing the Welsh nationalistic movement, Meibion Glyndwr (sons of Glyndwr) and the white eagle symbol used by the FWA.
Police are appealing for anyone with information or who may have seen anything suspicious to contact Narberth Police Station on 0845 330 2000.