This study aimed to identify novel antimicrobial peptides (AMPs) from Lactiplantibacillus plantarum K9 isolated from Protaetia brevitarsis seulensis larvae using genome and transcriptome analyses. L. plantarum K9 showed the strongest antimicrobial activity, with MIC90 values of 22 μl against E. coli and 21 μl against S. aureus. Approximately 54% of the antimicrobial activity was attributed to peptides or proteins. Genome analysis revealed a 3-Mb chromosome and three plasmids. Screening of hypothetical proteins using CAMPR4 identified 18 AMP candidates. Transcriptomic analysis showed that most AMP genes were more highly expressed in the stationary phase and were upregulated under low-pH conditions. Synthetic peptide assays demonstrated strong activity of AMP4, 6, 7, 12-2, 14, and 18 against S. aureus, while AMP16 and 18 were effective against E. coli. Among all candidates, AMP12 exhibited the highest activity and strong specificity toward S. aureus. Structural analysis revealed a conserved α-helical region that likely mediates membrane-targeting antimicrobial activity. These results suggest that LP AMP12 is a novel antimicrobial peptide contributing to the antimicrobial properties of L. plantarum K9.