Protein glycosylation, one of the post-translational modifications, is important for many protein functions, such as protein stability, folding and secretion. In the protein glycosylation, C-mannosylation was first identified in ribonuclease 2, and some proteins have been reported to be C-mannosylated; however, effects of its modifications for target proteins remain unclear. Hyaluronidase 1 (HYAL1), degrading hyaluronic acid (HA), has two predicted C-mannosylation sites at Trp¹³⁰ and Trp³²¹. In this study, we examined whether HYAL1 is C-mannosylated or not, and the effect of C-mannosylation on HYAL1. Using mass spectrometry, we first demonstrated that intracellular HYAL1 is C-mannosylated at Trp¹³⁰ but not at Trp³²¹. Surprisingly, although HYAL1 was secreted into conditioned medium and it possessed enzymatic activity, secreted HYAL1 was not C-mannosylated. Computer simulation demonstrated that C-mannosylation of HYAL1 at Trp¹³⁰ changed conformation of the catalytic active site, and faced Glu¹³¹ in the opposite direction toward its substrate, HA, indicating that C-mannosylation will negatively regulate its secretion, and will attenuate its enzymatic activity. Taken together, this is the first report that demonstrates the presence of C-mannosylation among HYAL family proteins, and our results suggest possible roles of C-mannosylation for secretion and enzymatic activity.