Pork production is closely linked to skeletal muscle growth and anabolic processes. This study investigated the effects of dietary supplementation with a multi-strain probiotic (Lactiplantibacillus plantarum, Streptococcus thermophilus, and Bacillus subtilis) on the growth performance, carcass traits, gut microbiota, and potential signaling pathways in growing pigs. A total of 144 weaning piglets (28 days old) were randomly allocated to two groups and fed diets with or without probiotics (0.1%) for 18 weeks. Pigs fed with probiotics showed significantly improved feed efficiency (p < 0.05) and greater muscle mass in the loin eye, arm shoulder, and blade shoulder regions. Microbiome analysis revealed significant enrichment of short-chain fatty acid (SCFA)-producing taxa, including Acidaminococcus, Allisonella, Dialister, and Megasphaera, alongside an increased cecal butyrate level in pigs fed probiotics. Integrated fecal microbiome and serum metabolomics analysis demonstrated that the metabolite profile was substantially altered by the supplementation of probiotics. Additionally, serum insulin levels, expression of the bile acid receptor tgr5, and upstream genes in the PI3K/Akt/mTOR pathway (igf1r, insr, and pi3k) were significantly upregulated (p < 0.05). Collectively, these results suggest that a multi-strain probiotic supplementation may be a promising strategy for improving muscle deposition and feed efficiency in commercial pig production.