Leitneria floridana Chapm., a shrub indigenous to North America, was long considered to be the sole member of the family Leitneriaceae, with uncertain affinities in and outside the class Amentiferae. Recent molecular analyses, however, show that the species is closely related to the genera Brucea J. F. Mill., Soulamea Lam., and Amaroria A. Gray in the family Simaroubaceae. Here, I report embryological features of L. floridana and provide anatomical corroboration for the molecular simaroubacean affinities of this species. I compared its embryology with those of the families Simaroubaceae, Meliaceae, and Rutaceae, which form a monophyletic clade in the order Sapindales. Comparisons showed that although Leitneria Chapm. resembled all three families embryologically, it fits best within the Simaroubaceae through possession of an unspecialized chalaza, straight and exarillate seeds, a non-multiplicative testa, a non-crystalliferous endotesta, and absence of a fibrous exotegmen. Within the Simaroubaceae, Leitneria also shares with Brucea the synapomorphies of a micropyle formed by an elongate, irregularly folded inner integument and a persistent tegmen composed only of endotegmen. Members of the Simaroubaceae are still poorly known embryologically, but available evidence supports the close affinity of Leitneria and Brucea.