Tannerella forsythia is a Gram-negative oral anaerobe which contributes to the development of periodontitis, an inflammatory disease of the tooth-supporting tissues leading to tooth loss. The mechanisms by which this bacterium colonizes the oral cavity are poorly understood. The bacterium has been shown to express two distinct sialidases, namely, SiaHI and NanH, with the latter being the major sialidase. Bacterial sialidases can play roles in pathogenesis by cleaving sialic acids on host glycoproteins, destroying their integrity, and/or unmasking hidden epitopes on host surfaces for colonization. In the present study, we investigated the roles of the SiaHI and NanH sialidases by generating and characterizing specific deletion mutants. Our results showed that the NanH deficiency resulted in a total loss of sialidase activity associated with the outer-membrane and secreted fractions. On the other hand, the SiaHI deficiency resulted in only a slight reduction in the total sialidase activity, with no significant differences in the levels of sialidase activity in the outer membrane or secreted fractions compared to that in the wild-type strain. The results demonstrated that NanH is both surface localized and secreted. The NanH-deficient mutant but not the SiaHI-deficient mutant was significantly attenuated in epithelial cell binding and invasion abilities compared to the wild-type strain. Moreover, the NanH-deficient mutant alone was impaired in cleaving surface sialic acids on epithelial cells. Thus, our study suggests that NanH sialidase might play roles in bacterial colonization by exposing sialic acid-hidden epitopes on epithelial cells.