Drosophila suzukii (Spotted Wing Drosophila) has recently become a serious invasive pest of fruit crops in the USA, Canada, and Europe, leading to substantial economic losses. D. suzukii is a direct pest, ovipositing directly into ripe or ripening fruits; in contrast, other Drosophilids utilize decaying or blemished fruits and are nuisance pests at worst. Immature stages of D. suzukii are difficult to differentiate from other Drosophilids, posing problems for research and for meeting quarantine restrictions designed to prevent the spread of this pest in fruit exports. Here we used a combined phylogenetic and bioinformatic approach to discover genetic markers suitable for a species diagnostic protocol of this agricultural pest. We describe a molecular diagnostic for rapid identification of single D. suzukii larva using multiplex polymerase chain reaction. Our molecular diagnostic was validated using nine different species of Drosophila for specificity and 19 populations of D. suzukii from different geographical regions to ensure utility within species.