In a previous work [1], 17 Roman ceramic fragments (bricks and tiles) from Sarmizegetusa were analysed for emphasizing some physical characteristics and thermal treatment parameters. The present study intends to answer a very frequent archaeological question: which are the raw materials used for manufacturing the Roman ceramic tegular materials? In this respect, six samples of clays from different locations in the vicinity of the former capital of the Roman Province Dacia were analysed. The interpretation of the X-ray diffraction patterns of the bulk clay samples and of the fine clay fraction (< 2 mm) evidences a relatively simple mineralogical composition: kaolinite, smectite/vermiculite, quartz, Ca-Na feldspars (albite–anorthite), K-feldspars (microcline), illite, micas, chlorites, calcite and hematite. Scanning electron microscopy allowed illustrating the well-shaped kaolinite plates. The mineralogical and chemical composition, as well as the mechanical properties allowed separating three different clay types: (1) plastic clay, (2) sandy clay and (3) carbonate-rich clay. The main characteristics recommend the plastic clay as the proper raw material for ceramics. To verify, the mineralogical composition (by using XRD and optical microscopy), and the technical characteristics of the plastic, kaolinitic clay, after thermal treatment at 800°, 850°, 900° and 950° C, were analysed. The brown-reddish colour after firing, the water adsorption (12.82–13.89 %), density (1.95–2.00 g/cm3) and apparent porosity (25.61–27.16 %) are very similar to the previous studied Roman bricks and tiles. This means that the same clay source could have been used for obtaining any type of ceramics, following specific shaping and firing patterns.