Knowledge of mechanical properties and failure mechanisms of biofilms is needed to determine how biofilms react on mechanical stress. Methods currently available cannot be used to determine mechanical properties of biofilms on a small scale with high accuracy. A novel microindentation apparatus in combination with a confocal microscope was used to determine the viscoelastic properties of Streptococcus mutans biofilms. The apparatus comprises a small glass indenter and a highly sensitive force transducer. It was shown that the present biofilm, grown under still conditions, behaves as a viscoelastic solid with a storage modulus of 1–8 kPa and a loss modulus of 5–10 kPa at a strain of 10%. Biofilm failure was investigated visually through a confocal microscope by dragging the indenter through the biofilm. It was shown that the tensile strength of the biofilm is predominantly determined by the tensile strength of the extracellular polysaccharide matrix. The combination of microindentation and confocal microscopy is a promising technique to determine and characterize the mechanical properties of soft materials in various fields of microbiology.