Citruses are one of the major fruit crops globally. Among Mediterranean citrus producers, Sicily (southern Italy) is renowned for its high-quality fresh fruit production. Phytophthora diseases are a serious issue for citrus production worldwide and Phytophthora nicotianae is a prevalent causal agent of root rot in most citrus growing areas globally and particularly in the Mediterranean region. This study reports the occurrence of Phytophthora inundata as a root pathogen of declining mature citrus trees in eastern Sicily in association with P. nicotianae. This is the first record of P. inundata on citrus in Europe and the Mediterranean region. The species was identified on the basis of a morphology and multi-gene phylogenetic analysis, which included the internal transcribed spacer, β-tubulin and cytochrome c oxidase subunit 1. Pathogenicity tests on citrus saplings showed P. inundata was a less aggressive pathogen than P. nicotianae. However, the co-inoculation of both species produced more severe symptoms than inoculation with a single species, indicating an additive effect of these two pathogens and suggesting that opportunistic secondary pathogens like P. inundata may have a crucial role in complex diseases.