Vascular wilt (Fusarium oxysporum f. sp. niveum [Fon]) is the main fungal disease of watermelon crops worldwide and can be caused by any of the four known races of this fungus (race 0, 1, 2 and 3). In Mexico, there are no reports of the presence of any of these races. The presence of plants with wilt and vascular necrosis has been observed in commercial watermelon plantations in the municipality of Hermosillo, Sonora, Mexico, mainly during the spring-summer growing seasons. The aim of this study was to identify the Fon race causing vascular wilt in watermelon plants in Hermosillo, Sonora. Three fungal isolates were collected from symptomatic plants. Based on their morphological characteristics, DNA amplification of the isolates (with specific primers Fon-1/Fon-2, FONSIX6F/FONSIX6R and FNR3-F/FNR3-R, which allow differentiating Fon race 1, 2 and 3) and inoculation into three differential watermelon cultivars, the three isolates (UESFON01, UESFON02 and UESFON03) were identified as F. o. f. sp. niveum race 1. To our knowledge, this is the first report where F. o. f. sp. niveum race 1 is identified as the causal agent of wilt, vascular necrosis and plant death in watermelon crops in Mexico.