A series of N-chloro-alkoxy-s-triazine antimicrobial additives were synthesized in this study. The structures of the samples were confirmed with FTIR, H-1 NMR, and thermal analyses. The N-chlorinated alkoxy-s-triazines provided potent antimicrobial activities against Staphylococcus aureus (S. aureus, Gram-positive bacteria) and Escherichia coli (E. coli, Gram-negative bacteria) under waterborne test conditions. The N-chlorinated alkoxy-s-triazines were incorporated into polyurethane, one of the most widely used polymer materials, as antimicrobial additives through solvent casting. Upon contact, the resultant films provided a total kill of S. aureus and E. coli, and they successfully prevented the formation of bacteria] biofilms on film surfaces, pointing to great potential of the new N-chloro-alkoxy-s-triazine antimicrobial additive approach for a broad range of antimicrobial and Biofilm-controlling applications.