The genus Acaciella Britton & Rose is reinstated. All species are neotropical with the highest species diversity along the Pacific coast of Mexico. The genus is characterised by three features exceptional in Acacia Miller: unarmed plants, lack extrafloral nectaries and pollen polyads 8-celled. With the exception of the number of stamens, Acaciella shares more features with the genus Piptadenia (tribe Mimoseae) than with Acacia. In terms of pollen and free aminoacids it is related to the pantropical Acacia subgen. Aculeiferum. Molecular studies place Acaciella sister to a monophyletic clade comprising elements of the mainly Australian Acacia subgen. Phyllodineae, and the pantropical tribe Ingeae. The 85 validly published names in Acaciella are referred here to 15 accepted species and five varieties. One new species and one variety are described, and two new names and five new combinations proposed. Twenty one names are lectotypified. Keys, descriptions, illustrations, distribution maps and conservation