Antimicrobial peptides (AMPs) are increasingly recognized for activities beyond bacterial killing, including modulation of virulence at sub-inhibitory concentrations. Here we examined a panel of synthetic α-helical AMPs originally designed against non-pathogenic Escherichia coli for activity against Salmonella enterica serovar Typhimurium. Under physiological salt conditions, these peptides exhibited antimicrobial activity against laboratory and multidrug-resistant Salmonella isolates. Using a prgH::gfp reporter, we found that several of the peptides reduced expression of the Salmonella pathogenicity island 1 (SPI-1) type III secretion system at concentrations below those required for growth inhibition. This effect occurred under conditions that did not affect bacterial growth, viability, or gross morphology, as confirmed by MIC determination, time-kill assays across growth phases, and electron microscopy. One of these peptides, the Leu/Ile-rich L1L, attenuated SPI-1 expression sufficiently to reduce epithelial cell invasion in gentamicin protection assays. Whereas the corresponding D-enantiomer and the Trp/Arg-rich L24 peptide showed less activity, indicating that stereochemistry and sequence composition critically influence virulence modulation independently of antibacterial potency. Direct comparison with polymyxin B demonstrated that L1L suppressed SPI-1 expression at a lower fraction of its inhibitory concentration, highlighting a greater separation between virulence attenuation and growth inhibition. Proteomic analysis revealed that L1L exposure induces envelope remodeling and antimicrobial peptide resistance pathways while repressing SPI-1 regulators and effectors, consistent with a regulatory shift from invasion toward stress adaptation. Together, these findings demonstrate that synthetic AMPs can exert potent, stereochemically specific anti-virulence effects against Salmonella at sub-inhibitory concentrations and underscore the importance of incorporating virulence-associated endpoints and benchmark comparisons into future AMP design and evaluation strategies.