Adult specimens of monorchiids (Digenea) were collected from the intestines of the white grunt,Haemulon plumieriiLacepède (Haemulidae), and the white mullet,Mugil curemaValenciennes (Mugilidae) from five localities off the Yucatán Peninsula and one locality in the Gulf of Mexico. Some specimens were photographed and sequenced for two molecular markers, the large subunit (LSU) of nuclear rDNA and the cytochrome c oxidase subunit 1 (cox1) of mitochondrial DNA. Other specimens were processed for morphological analyses. Newly generated sequences were aligned with other sequences available in GenBank. Bayesian inference and maximum likelihood analyses were implemented using the data sets of LSU andcox1independently. Reciprocal monophyly evidenced through phylogenetic analyses, sequence divergence values for both molecular markers, and detailed morphological analyses, including scanning electron microscopy photomicrographs, revealed three new genetic lineages, i.e., species, as parasites ofM. curema. The three new species areSinistroporomonorchis mexicanusn. sp.,Sinistroporomonorchis yucatanensisn. sp., andSinistroporomonorchis minutusn. sp. Two additional species of monorchiids were sampled, characterised molecularly, and re-described, namelySinistroporomonorchis glebulentus(Overstreet, 1971) from the white mullet, andAlloinfundiburictus haemuli(Overstreet, 1969), from the white grunt.