A new aphid species record for South Korea, Greenidea (Trichosiphum) prunicola, is reported based on the collection of apterous viviparous females from Castanopsis sieboldii in Tongyeong-si in 2024. This discovery increases the number of known Greenidea species in South Korea to four. Detailed morphological descriptions, measurements, host plants, and distribution data for G. (T.) prunicola are provided. The species is characterized by its glossy reddish-brown body, a body length of 1.70–2.60 mm, a siphunculus that is 0.32–0.37 times the body length, and long, primarily bifurcated dorsal setae. Previous host records include Prunus spp., and its known distribution now includes South Korea, China, and India. A key to the four species of the genus Greenidea now known to be present in South Korea is also provided.