The genus Alloophistolecithum was recently created and comprises species that were previously included in the genus Lecithaster. The two genera form the subfamily Lecithasterinae. The genera includes species that predominantly parasitize marine fish, and occasionally those inhabiting fresh or brackish water. This study describes a new species of Alloophistolecithum, which is a parasite of the Argentinian silverside (Odontesthes bonariensis), a fish species that inhabits brackish waters. Alloophistolecithum magaliae n. sp. is characterized by smooth, unnotched testes; a uterus extending to the posterior end of the body; and short caeca that do not reach the anterior edge of the vitellary. As genetic analyses are currently limited to a few species within the genus, it is premature to establish clear relationships between them. The Argentinian silverside is considered to be of great economic importance, having spread not only within Argentina but also to other countries for commercial exploitation. The anthropogenic spread of silversides outside their natural range could pose a risk of biological invasion, so studying their parasitological fauna is important.