Antimicrobial resistance is a major threat to global health and demands urgent action. In recent decades, the overuse and misuse of first-line antibiotics have increased bacterial resistance and narrowed treatment options. This problem is acute for small-molecule antibiotics, which often target specific active sites in bacterial proteins. Pathogens can quickly alter these sites, reducing drug efficacy. New therapeutic approaches are therefore needed to slow the emergence of resistant variants and counter rapid pathogen adaptation. Antimicrobial polymers (AMPs) offer a promising alternative because they kill bacteria by disrupting their membranes through electrostatic interactions. However, the cationic and hydrophobic features that support bacterial killing can also harm human cells, for example through hemolysis. Therefore new AMPs must be tuned to balance strong bactericidal activity with low toxicity by changing the type, ratio, and spatial arrangement of charged and hydrophobic groups. Most AMPs use amino-based cations that mimic those found in natural host-defense peptides. However, early signs of resistance to amino-based AMPs show the need for new cationic structures. Our work addresses this need by developing AMPs based on trivalent sulfonium cations.