Porphyromonas gingivalis, the major etiologic agent of chronic periodontitis, produces a broad spectrum of virulence factors, including outer membrane vesicles, lipopolysaccharides, hemolysins and proteinases. Antimicrobial peptides (AMPs) including bacteriocins have been found to inhibit the growth of P. gingivalis; however, these peptides are relatively large molecules. Hence, it is difficult to synthesize them by a scale-up production. Therefore, this study aimed to synthesize a shorter AMP that was still active against P. gingivalis. A peptide that contained three cationic amino acids (Arg, His and Lys), two anionic amino acids (Glu and Asp), hydrophobic amino acids residues (Leu, Ile, Val, Ala and Pro) and hydrophilic residues (Ser and Gly) was obtained and named Pep-7. Its bioactivity and stability were tested after various treatments. The mechanism of action of Pep-7 and its toxicity to human red blood cells were investigated. The Pep-7 inhibited two pathogenic P. gingivalis ATCC 33277 and P. gingivalis ATCC 53978 (wp50) strains at a minimum bactericidal concentration (MBC) of 1.7 µM, but was ineffective against other oral microorganisms (P. intermedia, Tannerella forsythensis, Streptococcus salivarius and Streptococcus sanguinis). From transmission electron microscopy studies, Pep-7 caused pore formation at the poles of the cytoplasmic membranes of P. gingivalis. A concentration of Pep-7 at four times that of its MBC induced some hemolysis but only at 0.3%. The Pep-7 was heat stable under pressure (autoclave at 110 and 121 °C) and possessed activity over a pH range of 6.8-8.5. It was not toxic to periodontal cells over a range of 70.8-4.4 μM and did not induce toxic pro-inflammatory cytokines. The Pep-7 showed selective activity against Porphyromonas sp. by altering the permeability barriers of P. gingivalis. The Pep-7 was not mutagenic in vitro. This work highlighted the potential for the use of this synthetic Pep-7 against P. gingivalis.