At Santa Filitica, a Roman settlement in north-western Sardinia occupied until the ninth century ad, archaeological excavations have found the remnants of a furnace consisting of a semi-circular base made of stony slabs and tiles bound with clay. The furnace is attached to a wall of Roman age. Layers of the sixth century ad, bearing several variously oxidized slags, were found close to the furnace. These findings first testify to an ironworks in Sardinia, within a well-defined context. XRPD and SEM–EDS mineralogical and textural analyses suggest that the slags derive from a bloomery and smithing work that was the first evidence of this type documented in Sardinia during the Early Middle Ages. Chemical analyses (performed with ICP and INAA) of rare earth elements and trace elements in two slags and in two Sardinian iron deposits allow some conclusions to be drawn on the local provenance of the ore. Our comparison of the Sardinian findings and some slags representative of archaeological smelting sites at the front of Elba island—the largest and longest lived iron-working sites in the Mediterranean—also improves the methodology with which iron slags derived from different ores are compared.