Medical devices provide an ecological niche for microbes to flourish as a biofilm community, protected from antimicrobials and host defenses. Biofilms formed by Candida albicans, the most common fungal pathogen, survive exposure to extraordinarily high drug concentrations. Here, we show that beta-glucan synthase Fks1p produces glucan, which is deposited in the biofilm matrix. The extracellular glucan is required for biofilm resistance and acts by sequestering antifungals, rendering cells resistant to their action. These findings provide the genetic basis for how biofilm matrix production governs drug resistance by impeding drug diffusion and also identify a useful biofilm drug target.