This review synthesizes the available evidence on the effects of Hermetia illucens meal inclusion on the intestinal microbiota of fish. A literature search was conducted in Scopus, Web of Science, PubMed, and Embase from inception to 23 April 2025, including in vivo experimental studies reporting changes in fish intestinal microbiota. Of 2525 records identified, 39 studies met the predefined PICOS-based eligibility criteria and were included in a PRISMA 2020-guided synthesis; risk of bias was assessed using the SYRCLE tool. Given the substantial methodological heterogeneity among studies (fish species, dietary formulations, sequencing platforms, and analytical pipelines), no quantitative meta-analysis was performed, and results were synthesized semi-quantitatively based on the direction of reported effects. At the phylum level, most studies reported significant increases in Bacillota (41% vs. 16% decreases; n = 32) and Actinomycetota (65% increases; n = 26), while Pseudomonadota showed a recurrent, though not universal, reduction (50%; n = 28), with enrichment of genera such as Bacillus (70%; n = 20), Enterococcus (81%; n = 16), and Oceanobacillus (93%; n = 15). These changes have been hypothesized to involve prebiotic, metabolic, and antimicrobial mechanisms related to chitin, lauric acid, and antimicrobial peptides. Overall, H. illucens meal inclusion (~10–30%) appears to be associated with a predominant, though not universal, modulation of the fish intestinal microbiota toward profiles with greater fermentative and chitinolytic potential; an inference based on the ecological attributes of the enriched taxa rather than on functional measurements. The magnitude and consistency of this modulation depend on dose, species, habitat, and meal processing type.