Phytophthora is the major genus within the Oomycota, which includes primary parasites of fine roots and causal agents of root and collar rots and bark cankers of hundreds of tree and shrub species worldwide. Since 2009, extensive dieback and mortality of Mediterranean oaks caused by root loss and root rot have been reported in Sardinia (Italy). Between 2010 and 2012, sixteen oak stands were investigated to establish whether Phytophthora species were associated with these decline events. Soil samples from declining trees were collected and baited using oak leaflets. In addition, bark samples were taken from lesions and cankers present on stems and roots. Isolations were made using SMA selective medium for Phytophthora. Isolates were identified using both morphological analysis and DNA-based techniques. Eight Phytophthora species were detected: P. cinnamomi, P. citrophthora, P. cryptogea, P. europaea, P. gonapodyides, P. psychrophila, P. quercina and P. uliginosa. These are the first records worldwide of P. citrophthora, P. cryptogea, P. quercina and P. psychrophila on Quercus suber, and of P. europaea and P. uliginosa on Mediterranean oaks. Phytophthora cinnamomi was the most frequently isolated species. The oak-specific P. quercina was detected at four sites where it was causing extensive dieback of both Q. ilex and Q. suber trees. The implication of these findings on Mediterranean oak forests health is discussed.