Eucalyptus (Myrtaceae) is a genus native to Australia and has become widely distributed due to its rapid growth and commercial applications. However, these trees are highly susceptible to pests, including gall wasps of the genus Ophelimus (Hymenoptera: Eulophidae), which can cause significant damage to plantations. In Argentina, two species of this genus have been detected: Ophelimus maskelli was first reported in 2013, and an unidentified species, Ophelimus sp., was observed in 2017. This study aimed to identify the species Ophelimus sp. present in Argentina using morphological and molecular data. Eucalyptus branches with leaves containing mature galls were collected from two locations in the Province of Buenos Aires. The leaves were stored under controlled conditions until the adults emerged. Specimens were preserved in 96% ethanol for molecular analysis or in 70% ethanol for morphological characterization. Molecular analyses included the sequencing of two molecular markers: Cytochrome Oxidase subunit I (COI) and the 28S rDNA region (28S). Morphological analysis was performed by critical point drying specimens preserved in ethanol and mounting them in cards and Canadian balsam. The results confirmed the first record of Ophelimus mediterraneus in Argentina and the Americas, affecting Eucalyptus species such as E. dunnii, E. viminalis, E. benthamii, and E. cinerea, and the first record of males of this species. The galls are circular and brown and develop on only one side of the leaf. The sequences of COI and 28S revealed a 99-100% identity with those of European populations of this species. In addition, this study provides the first description of O. mediterraneus males, contributing new insights into their morphology and biology.