Accumulated studies have revealed that spiders, which are believed to be true predators, also feed on various plant materials such as pollen, nectar, and stigmatic exudate. Hereby, we report observational cases of fermented tree sap feeding by four spider species, namely Sinopoda forcipata (Karsch, 1881) (Sparassidae), Otacilia komurai (Yaginuma, 1952) (Phrurolithidae), Weintrauboa contortipes (Karsch, 1881) (Pimoidae), and Doenitzius cf. peniculus Oi, 1960 (Linyphiidae), in deciduous forests in Japan. This is the first report of spiders feeding on fermented sap, and also the first report of plant material consumption in the families Sparassidae, Phrurolithidae, and Pimoidae.