The antibacterial activities of oligomer and polymer chitosan from different sources (shrimp, crab and squid) were examined against foodborne pathogenic bacteria. Sixteen tested foodborne microorganisms comprised of eight Gram-positive bacteria-namely, Listeria monocytogenes strains 101, 310, 108, Scott A and V7, Staphylococcus aureus, Bacillus cereus and Bacillus subtilis and eight Gram-negative bacteria-namely, Vibrio parahaemolyticus strains BCC 24339, FS 004, TDH 293, TDH 330 and FS 015, Escherichia coli, Salmonella Weltevreden and Salmonella Typhimurium. The minimum inhibitory concentration (MIC) of the chitosans was analyzed by an agar dilution method and the minimum bactericidal concentration (MBC) was analyzed by a broth dilution method. Polymer chitosan from crab showed the best antilisterial activity while polymer chitosan from squid showed the best efficiency for inhibiting S. aureus, B. cereus and B. subtilis. Oligomer chitosan from squid showed the strongest antibacterial activity on V. parahaemolyticus. However, S. Weltevreden was the most resistant to the chitosan groups (MIC = 0.15%v/v). In addition, chitosan with a degree of deacetylation (DD) 98.01% had the strongest antimicrobial activity against the growth of L. monocytogenes and B. cereus. The antibacterial activity against B. cereus was enhanced with increasing DD. Therefore, the antimicrobial activity of chitosan depends on the target microorganism, chitosan source, molecular size and DD. These observations suggested that the source, DD and molecular size of chitosan must be chosen selectively to control the target foodborne pathogens.