Several eriophyoid mite species of the genus Aceria induce gall formations on different Juglans species, causing significant economic losses and wood degradation. Aceria tristriata (Nalepa) and A. erinea (Nalepa) are well- established and considered the most injurious eriophyoids on walnut in Europe and Serbia. Recently, A. brachytarsus (Keifer), an invasive species native to the USA, was identified on J. regia and J. nigra in Hungary, which prompted investigation on Aceria spp. associated with J. regia in Serbia. Initial molecular analyses based on the mtCOI barcode region of mites collected from leaf galls of J. regia in Serbia, revealed the presence of three genetically distinct lineages. Two were identified as A. tristriata and A. erinea, whereas the third lineage remained unidentified. This study presents extended integrative analyses, providing new evidence regarding phenetic and genetic differences between these three lineages with the ultimate aim to resolve the taxonomic status of the third species. Eriophyid mites were collected from leaf galls of J. regia at multiple sites across Serbia. The mitochondrial COI barcode region and nuclear 28S rDNA from individual specimens were amplified and sequenced, to assess genetic variation of the species. Morphometric analyses of multiple populations corresponding to each genetic cluster, detailed characterization of gall symptoms on J. regia, and comparison with material from Hungary were conducted to integrate molecular, morphological and biological evidence. Morphometric analyses of multiple walnut gall mite populations clearly separate three morphologically consistent groups, corresponding to the previously defined molecular clusters. Molecular data and morphology of the studied species are identical to those of a population recently found in Hungary, whereas data on host range and gall type do not match records of A. brachytarsus in North America. Comparison of symptoms shows that the gall type induced by this species differs markedly from the symptoms typical for A. brachytarsus on J. hindsii and J. californica in its native range. These findings indicate that the third Aceria lineage on J. regia in Serbia and the corresponding Hungarian population represent a distinct species, despite morphological similarities to A. brachytarsus. Further research is required to determine whether this newly recognized species is native to Europe and to clarify its evolutionary relationships with A. erinea and A. tristriata using broader phylogeographic and comparative genomic analyses.