Simple Summary: This publication describes a new Lachninae aphid genus and species (Insecta, Hemiptera: Aphididae)—Miyalachnus—from Japan. This genus is characterized by unique and outstanding morphological features on the legs and antennae and can be easily recognized within the remaining Lachninae genera. We provided a detailed scanning electron microscopy study on the general morphology and sensilla of the viviparous and sexual generations. We presented the biology of the genus with descriptions of all available morphs. The tribe Tuberolachnini within the Lachninae (Hemiptera: Aphididae) is particularly intriguing due to its morphological traits and various ecological associations. Among the genera within this group, Pyrolachnus stands out as relatively understudied. Currently, only one species, Pyrolachnus imbricatus nipponicus Sorin, 2011, is known from Japan, distinguished by its distinctive characteristics. Through meticulous morphological analyses, we introduce a novel Lachninae genus, Miyalachnus gen. nov., associated with Cerasus and Prunus spp. (Rosaceae) in Japan. This new genus accommodates P. imbricatus nipponicus, now recognized as Miyalachnus nipponicus (Sorin, 2011) comb. nov. Additionally, we present a second species within this genus, Miyalachnus sorini sp. nov., along with comprehensive SEM morphological examination and insights into its biology. Our study describes in detail the morphological characteristics of both viviparous and bisexual generations of Miyalachnus, as well as their relationships with related genera. [ABSTRACT FROM AUTHOR]