Abstract Spider ants of the genus Leptomyrmex Mayr (Hymenoptera: Formicidae: Dolichoderinae) are conspicuous species of Australasian rainforests, with putative fossil relatives in the Neotropics and Europe. There is longstanding debate over the biogeographical history of the genus, with the Palaearctic and Neotropical regions proposed as alternate centres of origin. We propose a resolution of this debate with the recent discovery and analysis of an extant species from central Brazil, L. relictus sp.n. , which we describe from workers, males and brood. We sequence ten nuclear genes in the new species and in several Australian Leptomyrmex species, and append these data to a 54‐taxon, 10‐gene data matrix previously generated for the subfamily Dolichoderinae. We conduct phylogenetic and divergence dating analyses, and re‐evaluate the fossil record of the group. We recover Leptomyrmex relictus sp.n. as a member of the Leptomyrmex clade with high support. It is sister to the Australasian species, and the genus Leptomyrmex is, in turn, sister to a pair of Neotropical genera, Forelius and Dorymyrmex . We infer a Neotropical origin for the genus and estimate a mid‐Eocene (46 Ma, 95% CI 56 to 36 Ma) origin for the crown genus and an Oligocene origin for the Australasian clade (29 Ma, 95% CI 40 to 19 Ma). We confirm placement of the Dominican amber species † L. neotropicus Baroni Urbani in the genus but reject a close relationship with the Palaearctic fossil taxa † Leptomyrmula Emery and † Usomyrma Dlussky, Radchenko & Dubovikoff, considering them incertae sedis in the subfamily (Dolichoderinae). In contrast to the mesophilic preferences of the Australasian species of Leptomyrmex , the new Brazilian species inhabits cerrado (dry savannah). Our results support a Neotropical origin for spider ants with dispersal to Australia. Rafting on west‐bound currents and/or a historical diversity imbalance between Australia and South America are proposed as alternate hypotheses to explain a pattern of biased E–W mid‐Tertiary dispersal for ants with austral distributions. This pattern is suggested by our results in conjunction with observations of other ant clades. Overall, our findings highlight the value of integrated taxonomy, critical interpretation of morphology, and a comparative phylogenetic framework when conducting palaeontological and biogeographical studies of insect species. This published work has been registered in ZooBank, http://zoobank.org/urn:lsid:zoobank.org:pub:6E9E6617‐6E53‐40B8‐82C7‐67F89A83C553 .