To date, the monospecific Dendrorchis is represented by D. neivai, reported from Brazil and parasitizing Brycon lundi. Dendrorchis retrobiloba n. sp. is described from the swim bladder of Astyanax fasciatus from Cañada del Dragón stream in Montevideo, southern Uruguay (34°47′S, 56°14′W), and the emended diagnosis of Dendrorchis is given. The diagnosis of Dendrorchis has been modified to include the following characters: Phyllodistominae with flattened body, wider in the region posterior to acetabulum. Intestinal ceca sinuous or not. Testes branched or with superficial lobes. Ovary with deep or superficial lobes. Seminal receptacle present or absent. Vitellaria ellongate or pyriform. Uterus extends to posterior region and to portion of body anterior to acetabulum. Eggs oval, variable size, thin-shelled, without opercula. Dendrorchis retrobiloba n. sp. differs from D. neivai by having pyriform vitellaria, ovary larger than the testes and not deeply lobed, wider distribution of the uterus, smaller egg size, testes with superficial lobes, oral sucker wider than the acetabulum, shorter esophagus, 2 terminal lobes at the posterior end of the body, and much smaller body length. Despite being found in different hosts, both species of Dendrorchis parasitize the swim bladder of freshwater Characidae.