The long-snouted bat (Platalina genovensium) is an endemic nectar-feeding bat from arid regions in western Peru and Chile. In certain areas of its distribution, it is likely the only bat pollinator and seed disperser of plants, and it is currently highly endangered. However, some aspects of its ecology, diet, and pollinator role remain unknown, especially in the unique and endangered lomas ecosystem. We conducted monthly evaluations of bats and plants over 15 months in the lomas of the National Reserve of Lachay (Peru) by sampling in three different plant communities along the Guayabito gorge. We collected fecal and pollen fur samples from 16 individuals of P. genovensium and recorded the phenology of its potential plant food resources. We differentiated fecal sample components and then identified and counted all pollen morphotypes in order to estimate their frequency and abundance in the samples. We also analyzed the niche breadth. Our results showed that the dietary components of P. genovensium included nectar, pollen, fruits, and insects, and that it utilized the cacti community for feeding. Pollen of the genus Haageocereus (Cactaceae) was the most frequent and abundant resource recorded, which was available all year long; moreover, the niche breadth analysis revealed that P. genovensium has a specialized diet on the Haageocereus cactus in Lachay. Finally, we suggest that P. genovensium could serve as a pollinator for Haageocereus acranthus, Haageocereus pseudomelanostele, and Puya ferruginea (Bromeliaceae) due to the large pollen loads it carries and the specific placement of pollen on different body zones.