PURPOSE: This work described a new species of Ceratomyxa, based on morphological and phylogenetic analyzes of myxospores collected from the gallbladder of the fish Astyanax mexicanus. METHODS: Sixty-two specimens were captured, between December 2022 and February 2024, in the Flexal River, in the community of Tessalônica, state of Amapá. The specimens were transported alive to the Laboratory of Morphophysiology and Animal Health, at the State University of Amapá, where the studies were carried out. RESULTS: Eleven specimes of A. mexicanus was parasitized with Ceratomyxa tessaloniensis n. sp. Morphological and morphometric analyses revealed strongly arched spores that measured 3.13 ± 0.5 μm in length and 12.18 ± 1.9 μm in thickness with a posterior angle of 74º ± 10.6º and two polar capsules measuring 1.74 ± 0.1 μm long and 1.5 ± 0.16 μm wide. The partial sequence of the 18 S gene from spores of Ceratomyxa tessaloniensis n. sp., comprised 932 base pairs. CONCLUSION: Phylogenetic analysis grouped Ceratomyxa tessaloniensis n. sp., with strong nodal support for the exclusive clade of freshwater species, being a new species described for the State of Amapá, the first described in A. mexicanus and the first species with the presence of non-wormlike plasmodia in South America.