Lomechusoides umbrosus sp. nov. (Coleoptera: Staphylinidae: Aleocharinae: Lomechusini) is described, a socially integrated myrmecophile associated with Formica polyctena Foerster, 1850. The species is documented from Sweden, Finland, Estonia, Poland, Belarus, and Western Russia (incl. Ural Mountains) based on both newly collected and museum specimens. Field observations show beetles on the surface of host mounds and within nest material, indicating a close association with red wood ant hills. Morphological comparison of the new species and similar Lomechusoides species is provided, including an identification key for Lomechusoides species of Northern Europe, and an update to the global L. strumosus group key. A detailed account of differences between the new species and L. strumosus (hosted by F. sanguinea), for which it has until now been misidentified as, is made. Mitochondrial COI barcodes are provided to support the morphology and show low but robustly supported genetic divergence (0.7%) between L. umbrosus sp. nov. and L. strumosus, which are similar to divergences between other currently recognised species in the genus. Key diagnostic characters include the shape of the pronotum, colour and microsculpture of the head, pronotum and abdomen, dimensions of the antennae, genital structures, as well as the ant host. Our study clarifies long-standing confusion over records of "L. strumosus" from nests of the Formica rufa group and suggests that host specificity is prevalent within Lomechusoides. The holotype is deposited at the Natural History Museum Denmark.