Here I describe a sooty wing phenotype in offspring produced by a wild-inseminated female of Eurema mandarina (Lepidoptera: Pieridae) collected in Tanegashima Island, Japan in May, 2014. Seven (3 females and 4 males) out of 36 offspring had a sooty phenotype on their wings, which is likely due to a simple genetic mutation (most likely a single recessive allele in an autosome). The expression of this phenotype, restricted to the ventral side of the wings, was not uniform—more conspicuous in hindwings than in forewings, and the affected areas were variable among individuals. Microscopic observation revealed that the sooty phenotype was attributed to the increased proportion of pigmented scales.