Asian chestnut gall wasp, Dryocosmus kuriphilus Yasumatsu (Hymenoptera, Cynipidae), is an important pest of Castanea species worldwide. Adults emerge in summer and lay eggs in a few days. Severe infestations may reduce chestnut and timber production. It was first recorded in Europe in 2002 from Italy. In April 2014, it was recorded in Turkey for the first time in Marmara region (Yalova, north-western Turkey). Despite quarantine measures, infected chestnut saplings were sold from this region to several growers in western Turkey (Aydın and İzmir) in 2018. Although most of these saplings were removed and burned shortly after planting, some planted saplings could not be detected and some adult emergences may have occurred before the sapling removal, we had accepted hypothetically the Asian chestnut gall wasp was introduced in the region. Confirming this hypothesis, we detected the pest in a private chestnut orchard in 2020 in İzmir. Although, to date, we have not found it in adjacent natural chestnut stands, it will most probably increase in the whole region in the near future. As the largest private chestnut orchards of Turkey occur in this region, the Asian chestnut gall wasp invasion in the region will have serious socioeconomic consequences. Accordingly, we began to inform growers and foresters in the region, and to develop pest management strategies.