Specimens of a new fruit taxon, Alatonucula ignis, from the early Eocene Laguna del Hunco flora of Chubut, Patagonia, Argentina, have characteristics consistent with assignment to Juglandaceae. Each fossil fruit consists of a nutlet with an attached wing that varies from unlobed to trilobed. One specimen clearly shows that the nutlet has a single locule partially subdivided into four chambers; its counterpart contains a four-lobed locule cast. The basally partitioned locule with lobed seed indicates that Alatonucula can be placed within Juglandaceae; the overall structure of the fruits suggests affinities to subfamily Engelhardioideae, which typically have fruits consisting of a nutlet attached to a trilobed wing. Parsimony analyses of a morphological matrix for extant and fossil Juglandaceae revised from a previously published study demonstrate that 1) Alatonucula groups with Engelhardioideae; and 2) Paleooreomunnea, an Eocene North American taxon that includes a fruit type with a trilobed wing, is within crown-group Engelhardioideae. Alatonucula, along with fossil juglandaceous pollen from the Paleogene of Patagonia, indicates that Juglandaceae may have had an unexpectedly early presence in South America. Furthermore, these fruits represent a Laurasian biogeographic signal in the Laguna del Hunco flora, which contrasts with the predominantly Australasian signal highlighted in previous studies.