Powdery mildew is one of the most commom foliar diseases of cultivated and wild cucurbits worldwide. Six different fungal species have been reported to be associated with powdery mildew of cucurbits, such as Podosphaera xanthii and Golovinomyces cichoracearum. The aim of the study was to identify the causal agent of the disease in watermelon plants (Citrullus lanatus) cultivated in greenhouses of the State of São Paulo. DNA total was extracted using Chelex method and the ITS region was amplified via standard PCR reaction using the specific primers PMITS1 and PMITS2. A fragment of 700pb was amplified and the sequence showed 99% of identity with P. xanthii (accession number JQ728480). Morphological observation showed that the conidia were ellipsoidal-ovoid, had fibrosin bodies and a bifurcated germinative tube in the lateral position. These characteristics and the molecular analyses confirmed P. xanthi as the etiological agent of powdery mildew in watermelon plants in São Paulo State. Morphological and molecular characterization of powdery mildew on watermelon plants in São Paulo state