Loma psittaca, previously described as inhabiting the intestinal mucosa of an anadromous fish, Colomesus pisttacus, from the Amazon Basin, is reported as being found for the first time in a marine fish, the hybrid grouper (Epinephelus lanceolatus♂×Epinephelus fuscoguttatus♀), from Lingshui city, Hainan Province, China, expanding the geographical distribution and host range of this parasite. Numerous whitish xenomas (0.5–0.7 mm in diameter) of this new isolate of L. psittaca, were found distinctly in the muscle layer of the host stomach wall. Electron microscopic observations showed a monokaryotic nucleus in all developmental stages. Round or elongated multinucleate merogonial plasmodia surrounded by numerous mitochondria were observed initially, subsequently transforming into uninucleate sporonts through multiple fissions. Sporonts, each with a large centrally positioned nucleus, further developed into sporoblasts. Each sporoblast mother cell gave rise to two uninucleate sporoblasts by binary fission. Mature spores were ellipsoidal, measuring 4.0 ± 0.3 (3.7–4.3) μm in length and 2.2±0.2 (2.1–2.5) μm in width. Spores possessed a mushroom-like anchoring disk, a bipartite polarplast, isofilar polar filaments arranged in 12 to 14 turns in one row, and a trilaminar spore wall. The obtained partial SSU rRNA gene sequence of the new isolate was 1330 bp in length and showed 99.4% sequence similarity with an estuary isolate of L. psittaca previously reported in South America. SSU rRNA gene-based phylogenetic analyses demonstrated that the two L. psittaca isolates first clustered together, then formed a dichotomy that included the digestive tract-infecting Loma species, L. acerinae, with high support values within group Ⅰ.