Purpose: Surface properties of denture materials are of clinical importance since they may affect plaque accumulation and staining. This in vitro study investigated the roughness of 20 commercially available denture materials. Materials and Methods: Four denture base resins, nine hard lining materials, and seven soft denture lining materials were evaluated for roughness. Roughness measurements were made using a Mitutoyo Surftest SV-400, and the mean arithmetic roughness values (Ra) obtained were used for the comparisons. Results: The overall Ra values ranged from 0.7 to 7.6 µm. The denture base materials group exhibited Ra values from 3.4 to 7.6 µm, whereas the hard liners were from 0.7 to 4.4 µm. The autopolymerized and visible light-cured soft liners presented Ra values from 0.7 to 3.5 µm, while the heat-cured soft liners ranged from 3.5 to 4.0 µm. Conclusion: The roughness exhibited by all of the materials tested (Ra values greater than 0.7 µm) indicates that there is a possibility for plaque accumulation, since 0.2 µm is considered the threshold below which no further bacterial adherence can be expected.