EnterotoxigenicEscherichia coli(ETEC) causing post-weaning diarrhea (PWD) in piglets have a detrimental impact on animal health and economy in pig production. ETEC strains can adhere to the host’s small intestinal epithelial cells using fimbriae such as F4 and F18. Phage therapy could represent an interesting alternative to antimicrobial resistance against ETEC infections. In this study, four bacteriophages, named vB_EcoS_ULIM2, vB_EcoM_ULIM3, vB_EcoM_ULIM8 and vB_EcoM_ULIM9, were isolated against an O8:F18E. colistrain (A-I-210) and selected based on their host range. These phages were characterized in vitro, showing a lytic activity over a pH (4–10) and temperature (25–45 °C) range. According to genomic analysis, these bacteriophages belong to theCaudoviricetesclass. No gene related to lysogeny was identified. The in vivoGalleria mellonellalarvae model suggested the therapeutic potential of one selected phage, vB_EcoS_ULIM2, with a statistically significant increase in survival compared to non-treated larvae. To assess the effect of this phage on the piglet gut microbiota, vB_EcoS_ULIM2 was inoculated in a static model simulating the piglet intestinal microbial ecosystem for 72 h. This study shows that this phage replicates efficiently both in vitro and in vivo in aGalleria mellonellamodel and reveals the safety of the phage-based treatment on the piglet microbiota.