Surfaces coated with the naturally-occurring polysaccharide chitosan resist biofilm formation by bacteria and yeast. The coating is highly effective at retarding or preventing the formation of Staphylococcus epidermidis, Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa and Candida albicans biofilms under medically relevant conditions. For instance, the chitosan coating demonstrated a 5.5 log10 reduction in areal viable cell density compared to a control surface. Coatings impregnated with antimicrobial agents like chlorhexidine were found to reduce S. epidermidis areal viable cell counts by less than 1 log10 unit as compared to a control surface. Reduced biofilm formation was confirmed with confocal microscopy. Using time lapse fluorescence microscopy and fluorescent dye loaded S. epidermidis, the permeabilization of these cells was observed as they alighted on chitosan-coated surfaces. This suggests chitosan disrupts cell membranes as microbes settle on the surface. Chitosan offers a flexible, biocompatible platform for designing coatings to protect implanted devices from infection.