Recognition that the critically endangered mallee Eucalyptus sp. Cattai (Gregson s.n. 28 Aug 1954) is a distinct species has been complicated by close morphological similarity between it and other members of E. subgenus Symphyomyrtus section Latoangulatae series Annulares. Recent genomic evidence has demonstrated that it is distinct from other species. In this study, we provide E. sp. Cattai with the new species name, E. cryptica T.C.Wilson, S.Rutherf. & S.M.Douglas, and use genomic scans of adults and seedlings to assist in its description and support its conservation by identifying hybrids. Accompanying the description of E. cryptica are images, diagnostic illustrations and an updated part of the Eucalyptus key for the Flora of New South Wales.