Squash (Cucurbita maxima) is an essential crop cultivated throughout Japan, and viral diseases such as mosaic disease cause a tremendous loss of squash yield in this region. Although surveying and detection of infectious viruses are essential for controlling viral diseases, currently, no in-depth field surveys have yet been conducted. Here, we conducted a field survey in virus-infected squash fields in Okinawa Prefecture, Japan. A total of 138 samples, including 131 from squash and 7 from weeds, were collected from Ishigaki Island, Miyako Island, and Okinawa Main Island. Nine identified viruses were then investigated by reverse transcription-polymerase chain reaction (RT-PCR). Most samples were found to be infected with zucchini yellow mosaic virus (ZYMV), and a few were infected with papaya ringspot virus (PRSV) or both ZYMV and PRSV. No other cucurbit-infecting viruses were detected among the samples. In addition, ZYMV and PRSV were detected in cucurbitaceous weeds grown near squash fields, suggesting that these weeds may act as an infection source for these viruses.