Illinoia (Masonaphis) lambersi (MacGillivray, 1960), a macrosiphine aphid species that mainly lives on Rhododendron spp., is native to western North America, but adventive in Europe, Macaronesia, South America, and West Asia. This species is regarded internationally as distributed in Japan, but no reliable literature records its distribution in Japan. The present paper, for the first time, formally reports the occurrence of I. lambersi in Japan and East Asia based on specimens collected in 1983 in Sapporo and 2020–2021 in Pippu, Hokkaido, Japan. Rhododendron molle subsp. japonicum was a principal host plant for I. lambersi in Hokkaido. Rhododendron albrechtii and R. × mucronatum were newly recorded as hosts for I. lambersi. Fundatrices were found in the populations in Pippu, indicating that the populations are holocyclic, as reported also in western North America.